I think it started when Bill Jemas came aboard Marvel and started to fan the flames of the rivalry again, and Quesada jumped in as well.
Bendis was just a little miffed because he was planning to do a Batman/Daredevil crossover that didn't happen because of the "feud" between DC and Marvel. He also said he wasn't expecting to ever work at DC after getting in an argument with a DC executive about it .