In the section on movement types, it states:
Switching from one movement type to another requires ending your
action that has the first movement type and using a new action that
has the second movement type. For instance, if you Climbed 10 feet to
the top of a cliff, you could then Stride forward 10 feet.
So, while not disrupted as defined in the rules, it still effectively ends the action as you cannot Stride from prone. So, you must end your Stride movement type in order to switch to the Stand Up or Crawl movement action.
Further. the side bar on pg 462 (In-Depth Action Rules) says this under Simultaneous Actions:
You can use only one single action, activity, or free action that
doesn’t have a trigger at a time. You must complete one before
From this, as well as the logic in YogoZuno's post (that the prone condition takes place in the square the target is in when the AoO takes place), we infer that the current Stride action will effectively end with the trip. If the Stride action is part of an Activity, the rest of the activity may be lost as well. From Activities (pg 461):
If an activity gets interrupted or disrupted in an encounter (page
462), you lose all the actions you committed to it.
Even though the trip does not disrupt the action, it does possibly interrupt it. This term is not clearly defined in this context. If simply using an reaction triggered by a action caused the action to be lost, then the disruption mechanic would be superfluous.
I would guess that the intent of the "interrupt" verbiage here is most likely the result of a voluntary interruption or the result of circumstance - the caster chooses to stop casting, prerequisites of the action are no longer met, etc. In this case, by strict RAW, the remainder of the Activity would be forfeit since the Stride action has been made impossible due to circumstance, even if there was, e.g., a target within reach for the Strike component of a Sudden Charge.
I have trouble imagining that this is RAI for all cases, and feel that the actual intent is that if the Activity was composed of sub actions (allowed additional movement of the correct type - Crawl or Stand Up - or an action that could be carried out from the target's current position and while prone), then the remaining actions it grants should be salvageable. In other words, the rule makes sense for something like Casting a Spell - there is only one discreet result, but an Activity that has a number of sub-actions should be able to have part of it interrupted without losing the whole thing. Obviously, this will be dependent on your table and your GM's interpretation.