I noticed this happening more often.
Last week it happened in a Black Temple run on one of my Alts. More than half the group just bailed after the 5th boss, even the tanks. I felt bad for the guy who started the raid. It was a pug and I didn’t know him, but he was bummed.
I was on my DK, so I started tanking, and the few who remained continued. It was fun - we were a plucky little group of leftovers, and we got it done.
But if I didn’t tank, it would have fallen apart completely. I’m sure that happens.