Without camouflage/hide in plain sight, the treant also requires cover or concealment in order to hide—if the victims are out in the open, it will necessarily break cover and no longer be hidden or eligible to even try to hide again until it reaches new cover/concealment. This concern is eliminated by camouflage or hide in plain sight (the shadowdancer’s version of it, anyway—not all forms of hide in plain sight include cover/concealment as part of their effects).
Likewise, without hide in plain sight, the treant cannot hide while being observed. So if it breaks cover/concealment, or misses a Hide check, or otherwise becomes no longer hidden, it can’t become hidden again. It may even lose the benefits of being hidden just by coming close enough to the victims to trample—the rules aren’t really clear on what counts as “observing” and whether or not you can “observe” something that was hidden from you (without making the Spot check). Again, hide in plain sight obviates this.
Even with camouflage/hide in plain sight, the victims are absolutely going to be aware that they’re being attacked; that’s unavoidable. So even if the treant is “hidden,” they can act to defend themselves—buffs and wards, area effects, and so on.
Furthermore, attacking—including trample—applies a −20 penalty on your Hide check. A Huge creature like a treant takes an additional −8 penalty on all Hide checks. So we’re talking about a truly colossal Hide check here.
But, with camouflage/hide in plain sight, and a truly colossal Hide check, and no special senses in play, the treant can remain hidden while trampling its victims. Those victims would be aware of its presence and the effects of its attacks against them, and would be able to respond accordingly, including by blind-firing attacks in the direction they think the treant ran.
If they fail to accomplish all of that for whatever reason, they would be seen when they attack, if not sooner. They could attempt to hide again afterwards (though without hide in plain sight they’d need to break line of sight to do it), but until they do they’d be vulnerable to counter-attack, e.g. from readied actions. Readying an action for “as soon as I see it” is a pretty classic use of a readied action, after all.