It happened sometime in Wrath. I remember the first time seeing something like that was in ToC, in a PuG of course. We wipe to Twin Val'kyrs once and everyone leaves. It's been like this ever since. No wipes in BWD until Nefarian, everyone leaves on first wipe. No wipes until Putricide, one wipe, game over.
This week takes the cake above all else though. I joined a Firelands HC, we just downed Staghelm, and all of a sudden some random dude who hadn't spoken once before that just said "I don't think we can do Ragnaros", and with that everyone left. I've never been as mind blown before, I haven't seen people be so... I don't even know what to say, it just blew my mind so much. I mean, we had a good group, and we didn't even fucking try? Everyone just follows it? It's just... NO.