Why did Gandalf let go and fall off the cliff?

[Orome]

In the movie it looks like he lets go but he really just slips off. Though I think he didn't fight it. He probably could have pulled himself back up but then he would be endangering everyone else.

what makes you think he could have pulled himself up? he was weak from trying to lock the door against the balrog, and the counter-spell. plus the balrog was kinda attached to his leg, i think