Karthis mentions in a recent post on Of Teeth and Claws, breaking crowd control successfully is more of an art form than you might think.
First, there is the concept of threat. Each method of crowd control angers a monster to varying degrees. Add to this the other abilities a crowd controller might have used previous to controlling the mob -- a hunter plinking away at the mob a few times to get its attention, for example -- and the CC'ers threat is pretty darn high. The monster, once they are free, will have eyes (and claws) only for the group member that kept them tied down in the first place. It makes sense, honestly. I know if someone shackled me to the ground for a minute or so I would be pissed.