I had this question in my game and the GM wasn't buying what was said here so I posted on another site: http://rpg.stackexchange.com/questions/39613/is-swarm-attack-an-action#3962 1 for more information.
The response is saying that the Swarm Attack is part of the move so it happens automatically any time they end their movement. The argument is made that it is written as the damage happens "at the end of their move" meaning that damage is part of the move and not an attack that happens after a move.
I still feel like the description is saying that it is not a Standard Melee Attack then describing how it is different (no attack roll needed). Nowhere does it say that the Swarm Attack is not an action or that it happens as part of the move action. Normally when attacks happen with a move action they are noted as a full round action such as with a Charge.
It would be nice if it explicitly stated that a Swarm Attack happens as part of the move - it would be described as special swarm movement instead of "Swarm Attack" that does auto damages at the end of their movement.