AOOs are resolved before the action triggering them; that does not mean in "game time" they occur before the triggering action.
It's not explicitly stated what happens if you use an AOO to trip somebody that is attempting to stand up from prone. I think it's utterly daffy to allow the folloing scenario:
1) PC1 (on ground) I'm getting up to kill that ORK.
2) GM OK that provokes an attack of opportunity. [roll, calculate] He trips you again.
3) PC1 Oh shucks, the ORK tripped me while i was lying on the ground. That sucks. Ok, well after he tripped me while i was lying on my stomach, knocking me prone (again), I finish getting up. I'm standing, right?
4) GM (huh?)
"Common sense", barring a direct rule to the contrary, should be the guide here.
If you trip somebody using the AOO from their standing up, the "trip" occurs at some point in the process of their standing (when they are on one knee, or 99% standing, or propped up on an elbow). If the trip works, the unlucky fellow is prone again, and has used up a move equivalent action.
Otherwise, well, you are allowed to trip somebody that is prone for something they have not even started yet (standing up) or letting them "double dip" and stand up twice (trip occurred when X was 99% standing, then X goes prone, then X stands up fully, which is a lot of up and down for one MEA). Whatever.