Specific trumps general.
Generally you can't take a free action except on your turn.
Specifically you can take a free action to speak on someone else's turn.
Specifically Mad Monkeys can take a free action to disarm on someone else's turn.
Are they doing it when the creature begins its turn in the swarm? If so, that would have to be an immediate action, not a free action.
Free actions aren't necessarily on your turn only. For example: "In general, speaking is a free action that you can perform even when it isn't your turn." And: "While you can’t take most free actions off your turn, Grab, Trip, Pull, Push, and Rock Catching’s free actions can all be used off-turn. "
Immediate actions have a once-per-turn restriction, so if it was an immediate action it would make the monkey swarm a lot less dangerous.
Isn't this just a rather poorly worded way of saying that the swarm can make as many "immediate" actions as there are separate targets starting their turns within the swarm, which yes is generally against the rule of being limited to one swift or immediate action per turn. Since we are talking swarms of many critters, action economy does't exactly apply.
Action economy applies exactly the way it does with any other creature. The action in question is a free action made at a specific time--hardly the only time a free action can occur on a different creature's turn due to specific wording.
If it were an immediate action, that would have other rules implications--for example, the swarm couldn't take any other (different) immediate actions that turn nor use its swift action on the following turn. (It's unlikely that the swarm has any such actions available, but it is certainly possible.) The design team could have chosen one way or the other; they landed on free action.
The monkeys attempt one disarm or steal combat maneuver each turn as a free action against any creature that begins its turn in the swarm, ...
This action does not occur as an immediate action on anther's turn. All these attacks occur on its own turn as free attacks. This is in addition to its swarm attacks as a monkey swarm.
I can see the attack maybe failing if the creature moves out of range before the monkeys can attack.
/cevah
The monkeys attempt one disarm or steal combat maneuver each turn as a free action against any creature that begins its turn in the swarm, ...This action does not occur as an immediate action on anther's turn. All these attacks occur on its own turn as free attacks. This is in addition to its swarm attacks as a monkey swarm.
I can see the attack maybe failing if the creature moves out of range before the monkeys can attack.
/cevah
Redirected emphasis.
You could stretch it and say that you had to remember each creature that begins its turn in the swarm's area and then do the disarm/steal when the swarm's turn comes up, but that has two icky consequences:
1. You have to remember who began their turn in the swarm back when it was their turn. That could have been processed and forgotten half an hour ago in a large and involved combat.
2. Each creature being disarmed/stolen from may have (in fact, usually will have) moved out of the swarm between the beginning of its turn and the swarm's turn, making the maneuver either "fizzle" or make no sense whatsoever.
Given that, it seems likely that design meant that the maneuver occurred at the start of the creature's turn, when its position in relation to the swarm is checked. It's still a free action and not an immediate action, though, for the simple reason that the text says so.
I agree that RAW you get those icky consequences.
For #2, that is why I mentioned the attack maybe failing.
As a GM I am fine with it happening at the beginning of each creature's turn instead. Reason: simplify combat and go with RAI.
/cevah