It's true for the PUGs, too. Generally whenever something goes wrong, people look for someone to blame. Of course they don't wanna blame themselves (and by extension, the zerg that they're a part of) so they just lash out at "Dumb puggies" for "doing something wrong".
Of course if the puggies do something right, it's only because "the zerg carried you" or something. But if you don't go with the zerg you're "doing it wrong". Meh. It's just lose/lose all around.
PvE - dungeons especially - are frankly just exhausting. The amount of running and dodging just makes it all lose meaning after a while. Especially when things have 10x more HP than they should...you're just wishing it would all get over with already.