Okay, so there's this thing in physics called the anthropic principle, which says that we can predict fundamental constants and lucky resonances of the universe if we can determine that, if they weren't that way, then humanity as we know it wouldn't exist and we couldn't ask the question "why are they that way?" in the first place. It's not the most satisfying answer to "why is this here?" but it is an explanation and it does have useful predictive power.
If you're a GM with a story-heavy narrative, people will be attached to their characters and it falls on you to implement the anthropic principle: characters generally won't die every encounter. Usually you do this by giving a forest of easily-survivable paths for the players to run around in. However, often there is some form of cliff edge near the forest, and sometimes some player insists on driving the Jeep straight for that edge at top-speed. Then you need to know your anthropic options. I see three big ones: the Jeep crashes in the forest, the Jeep crashes at the bottom of the cliff, and the Jeep's driver gets spontaneously ejected by Fate, causing the remaining characters to decide whether they want to stop it before it careens over the cliff. Let's take those in turn.
Your first option is that this is a forest: the ground is uneven. Stepping outside of the metaphor: the real world is complicated and the players do not always succeed at what they're doing. In your hypothetical scenario?
Game Master: The Wookiee chief says "no way in hell."
Brash Player: Screw it. I press the button.
GM: The big Wookiee in front of you recoils, cringing, as a click sound comes from
your hand and a flash of light blinds you and an awful sound deafens you.
Everything goes black. You feel some pressure against your chest.
BP: And, what, everybody is dead?
GM: You don't know, you're blind and deaf and the wind's been completely knocked out
Other Player: What do the rest of us see?
GM: You in particular were not blinded, because his body was in the way, but you
still got knocked down and you hear a loud ringing in your ears. You see a
Wookiee rush and tackle BP.
BP: I pull out my knife and stab him.
OP: What do I see happening there.
GM: OP, you see a lot of blood spatter as the Wookiee pins down BP, he appears to be
flailing wildly with a bloody stump for an arm. While you're looking so intently,
you feel a Wookiee crash into your side, tackling you to the ground.
OP: Is there anything I can do right now?
GM: I mean, there's a lot you can do, but you do have a 400-pound furry creature
pinning your arms down. You both feel them taking off your clothes.
OP: What?! I resist. STOP! THOSE ARE MY CLOTHES!
GM: Your hearing has recovered just well enough to hear the Wookiee chief say "no, we
can't take the risk, take off their clothes." There's also something about how no
warrior worth anything would conceal their weapons, some Wookiee culture thing.
BP: Why didn't that detonator take out the whole room?
GM: What is your character doing to try to answer that question?
Here we're invoking the anthropic principle with the assumption that the detonator will kill everybody in the room: given that this doesn't happen, the detonator must not have gone off (or at least, not completely gone off), and we reason about what must have happened.
The above dialogue exemplifies that you don't need to explain it just yet. Going back to the Jeep and the cliff: you don't explain why the rock was there, why the driver didn't see it, or how the rock caused the Jeep's right two wheels to go airborne, causing the Jeep to flip over entirely. At least, not immediately. You can determine that stuff after the session is over, or while it's ongoing. There are lots of reasons why it might have been impossible to see. (Maybe later we find out that this character had been a jerk to his weapons-supplier, who gave him known-to-be-somewhat-defective thermal detonators.)
Your second option, if the character has received three or four of these warnings and hasn't stopped (and the rest of the party hasn't ousted him) is to simply let them drive off the cliff. Sure, it violates the anthropic principle a little, but there's lots of viable nuance here, anyway. If the cliff is high enough, sure, kill the character. Optionally leave the rest of the party maimed, but feel free to kill all of them, too. Feel a certain pleasure in this, as of the GMs of yore, who would wax wistfully on about their TPKs -- "total player kills" -- where they annihilated a whole party. Your responsibility with a GM is to, every day, confront these players with a force that only has 50% or 75% of their strength, trying to overwhelm them: if you succeed in the face of those odds then you win. The monsters have families too, after all: only you will speak for them. In this case, the players are giving you this rarest of easy wins when they could have done something sane and obvious, but instead they committed suicide.
I said I'd be "anthropic" above, and so here's how to play that: if it's too unfair to the other players, or if you haven't given them enough warning, give them one last explicit warning to jump out of the Jeep before it reaches the cliff. Heck, go so far as to allow metagaming:
GM: The Wookiee chief says "no way in hell."
BP: Screw it. I press the button.
GM: OK, pause for a second. We haven't heard from everybody else here. You've all seen
that the situation has been rapidly deteriorating, what have you been doing? Have
you been backing away, running to take cover, or are you standing firmly beside
BP when he's pressing this detonator.
OP: Um... I think I would have been slowly backing away, and when he said "no way in
hell" I probably would have broken out into a sprint away from BP. It's not the
first time he's gone all-in on a bluff.
GM: Good, that might help save you. Anybody else?
OP2: While BP was focused up ahead I probably readied my weapons, including that
Gungan Energy Shield that I got from Those Movies That Shall Not Be Named.
GM: I'll allow it.
BP: Wait, are we just making stuff up now?
GM: No, it's a small personal item which belonged to OP's grandmother, who said that
it saved both her and her husband once... I forget the whole story. She's had it
all along, it's just not been terribly useful until now. Are you standing behind
it or cowering behind it?
OP2: Um, when he said "no way in hell" I probably started to cower behind it.
GM: That'll take you down to third-degree burns, yes.
BP: You're serious, you're going to let all of them make up whatever reason to live
while you kill me off?
GM: I'm not killing you off. You're committing suicide with a thermal detonator.
I'm just trying to figure out how many of your friends you took with you. Maybe
I'm going easy on them because they didn't expect you to actually do it.
Finally, if the story is too important to Fate, Fate can eject the driver's seat:
GM: The Wookiee chief says "no way in hell."
BP: Screw it. I press the button.
GM: You choose to press the detonator, but your thumb doesn't listen. Suddenly your
eyesight is a wash of colors and light.
GM: You saw some light, like blaster fire, but your eyes were turned forward, what are
BP: I turn around to face the blaster fire.
GM: You'd love to but your legs don't seem to be listening either. Your knees buckle
and you fall down onto them. Guys? A live thermal detonator is still in his hand,
and if it hits the ground it could explode.
OP: Eff this. I grab the detonator and deactivate it, then hold my hands up in
BP: I demand to know what the hell happened.
GM: You feel too woozy to let any words come out of your mouth, but reluctantly, a
"What happened?" comes out of your mouth.
OP: Can I see?
GM: The Wookiees behind you appear to be high-fiving some Wookiee with a bowcaster.
Come to think of it, that explains the metal protrudance coming out of the middle
of BP's upper back. You strongly suspect that she's a crack shot and that his
nerves are damaged.
If it's not a sniper round, it can be a heart attack induced by the stress of the situation, or a stroke, or an unexpected Jedi who was living among the Wookiees cutting the guy's freaking hand off. Of course you shouldn't use these gimmicks in general, but if the players are going to be "saved by Fate" anyway, you might as well get creative in how Fate saves them.
Don't get me wrong -- what you did is not too awful, you let the Jeep careen over the cliff while everyone inside was miraculously saved. But it's not very sustainable: you are implicitly encouraging the players to constantly drive off every cliff just to see what you'll do about it, and then you feel cheated because you spent so much effort on the forest and had to improvise what was at the bottom of the cliff. There are other options when you're stuck with this sort of acute "everything is coming to a head" situation.