Alright, this is a case where Jeremy Crawford's mantra applies to all Rules as Written answers.
Spells do Exactly What They Say
7th level transmutation
This spell reverses gravity in a 50-foot-radius, 100-foot high cylinder centered on a point within range. All creatures and objects that aren’t somehow anchored to the ground in the area fall upward and reach the top of the area when you cast this spell. A creature can make a Dexterity saving throw to grab onto a fixed object it can reach, thus avoiding the fall.
If some solid object (such as a ceiling) is encountered in this fall, falling objects and creatures strike it just as they would during a normal downward fall. If an object or creature reaches the top of the area without striking anything, it remains there, oscillating slightly, for the duration.
At the end of the duration, affected objects and creatures fall back down.
Gravity is reversed, things fall up. Cool. You're either falling to the ceiling or hanging from the floor. Right? Alright, we all agree here.
9th level evocation
Blazing orbs of fire plummet to the ground at four different points you can see within range. Each creature in a 40-foot-radius sphere centered on each point you choose must make a Dexterity saving throw. The sphere spreads around corners. A creature takes 20d6 fire damage and 20d6 bludgeoning damage on a failed save, or half as much damage on a successful one. A creature in the area of more than one fiery burst is affected only once.
The spell damages objects in the area and ignites flammable objects that aren’t being worn or carried.
Flaming orbs appear and plummet toward the targets. It doesn't say:
- Which gravity it used
- Where the orbs appear
- What inertia they already have from wherever they were pulled.
They appear and plummet toward the targets. If that means they appear "beneath"
them and fall up or above them and fall down or if they magically fall sideways -- because magic -- then they fall sideways. Whatever direction they plummet, they plummet at the targets.
So, if you're hanging from the floor or fell to the ceiling, maybe they appear "above you" closer to the floor than you currently are and traveling in your direction, or they appear already having a huge amount of inertia to overcome the effects of the reverse gravity spell. I don't care what you do narratively to convince yourself and your players why it happens -- the spell says what it does.
Points, not Creatures
The "a point you choose" don't have to be on the ground. It can be in a person's gut or head or whatever. The spell is instantaneous so the person won't have moved by the time the meteor gets there. Unless they have a reaction to use a spell to get out of the way, which, I suspect, is one of the reasons the target is points and not a creatures. Another is so that you can use it on objects. You can't magic missile or eldritch blast a door, but you can meteor swarm it. (Did this really need saying?)
If you're a strict follower of Rules as Written, or even if you aren't, part of the job a DM has is to make the rules make sense in context. Find a flavor or spin that makes the game mechanics work for the understanding of the surrounding environment and physics. So those meteors already have inertia from wherever they were conjured from that is greater than the reverse gravity field, or they conjured closer to the floor so they plummet "upward" toward the characters. "Physics" isn't a good reason to make someone, NPC or PC, lose a 9th level resource for nothing... However...
If reverse gravity is already up, and meteor swarm is cast -- no I'm not going to let a 7th level spell mangle a 9th level casting for no good reason because I think our world's physics should apply to a world where where flaming orbs can just appear, a person waving their arms and saying words can reverse gravity, an expensive enough gem can be used to bring back someone from the dead, and dragons are a real threat to local commerce... D&D is simulations, and our descriptions of what happens as a result of the mechanics is what makes anything in the world believable or not.
Rules as Cool
The rules aren't everything. If a PC readies a held casting of reverse gravity with a trigger of meteor swarm, that's pretty clever. And the order can make a narrative to why the "physics" works and rewarding the player for his creativity and his risk of wasting a 7th level slot if meteor swarm isn't cast is a perfectly acceptable narrative tool. Don't have an NPC do it, but allowing a PC to do that once is not a problem. That said, they'll have to fall 85 feet to get out of the way -- which will still be a huge amount of damage.