Spatula said:The swarm can make up to 3 such attacks, and would probably continue to make them until it successfully drags someone down. Then it attacks with the damage bonus (if it has any actions left), and its companions play pile-on. The way that multiple needlefang swarms complement each other is rather frightening. It's a neat image, but the damage done is a bit out of line for its level...
I would expect the swarm to always make one attack per round--it's not a brilliant tactician, after all. However, it would try to drag its victim down at least once, and twice if it can manage it.
As you say, it's when you have multiple needlefang swarms that it gets really ugly, because then it's virtually guaranteed that one or another of them will make its roll to put you on the ground. And then you eat a 2d10+4 aura attack from all of them at the start of your turn...