If you are Striding, and you trigger an attack of opportunity from an enemy wielding a flail, the flail's critical specialization can cause you to fall prone. If this happens, is your Stride action disrupted?
The prone condition says the only move actions you can take while prone are crawl and stand, but it's not clear how that rule is enforced when you're halfway through a Stride action that you originally met the prerequisites for.
In short, the target would be prone in the space they moved to and not be able to move any further, effectively ending the Stride action.
From the PF2 SRD (emphasis mine)
Some reactions and free actions are triggered by a creature using an action with the move trait. The most notable example is Attack of Opportunity. Actions with the move trait can trigger reactions or free actions throughout the course of the distance traveled. Each time you exit a square (or move 5 feet if not using a grid) within a creature’s reach, your movement triggers those reactions and free actions (although no more than once per move action for a given reacting creature). If you use a move action but don’t move out of a square, the trigger instead happens at the end of that action or ability.
So, firstly, it's clear from this rule that the trigger occurs after exiting the square moved from, meaning the provoking character will fall prone in the square moved to (note that this is different to 1st edition PF).
Next from the rules for the Prone condition.
The only move actions you can use while you’re prone are Crawl and Stand.
So, even though the target is in the middle of a Stride action, being Prone is now preventing them from using this action. Therefore, the action ends. The target could now use a new action to Crawl or Stand, assuming they have any actions remaining.
I do not think the rules are directly explicit on this matter.
However, I would rule that the creature would fall prone in the square they were moving into when the attack was triggered, or in their current square if not moving, and would cease striding having spent the action. I feel this is more similar to how it would work in reality* - we don't get tripped, walk another 20 feet, then fall over. Nothing about the stride action requires that you move your full distance.
This would work much like a critical attack of opportunity, or a readied attack that successfully grapples or disarms (with a flat check on grapple), which can interrupt spells mid-cast, with the target having spent their actions.
*Of course, in reality, if you were running and where tripped inertia would likely result in you flying a short distance in the direction you were moving when the trip occurred - however this would not be a continuation of your 'stride', as you would have relatively little control over the direction moved.
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 beginning another.
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.
