DPOS is all about creating a system that gives people financial incentive to do the right thing and maintains the ability to 'fire them' quickly if they don't with 100% certainty of getting caught.   The delegates have no power *BUT* to do the right thing.   

Can they collude?  Assuming they did collude the only thing they could do is block transactions and it would be instantly detected by all peers.   A fork would be launched immediately with these delegates stripped and the two chains would compete against each other in the market.  Assuming the delegates were just random people that attempted to screw with the system then consensus would be maintained.