I figured it was that 'flock-bonding' thing - similar to contact calling to make sure everyone is OK, even though they are out of sight. I guess it might also be the reverse of the saftey response that keeps them quiet when everyone else is quiet: in the jungle you don't want to draw attention to yourself by being the one making all the noise - but if everyone is shouting, you can shout too...
hmmm i never really thought about this but its a very good point.