Ugh, that's pretty awful for helping to carry the team through that mess.

Anyway, I feel a lot of times that DPS develop a sort of tunnel vision. Experience with other classes helps alleviate it, but they're still not required to watch party members the same way as healers are. As long as the tank and healer are up, they don't have to pay attention to anything except their own rotation and dodging. It takes something like the end of the fight or a dead tank to snap them out of it. More likely than not, he had those blinders on and assumed you let the tank drop dead when they actually dced.