Yeah, I've also had this experience a lot, especially in lower keys.
One place I've seen it several times is the first boss in Plaguefall. Someone keeps killing themselves by eating his barfs, or consistently fucks up some other way, and then just gives up and leaves.
My favourite is this DOS run I had. It was a few months ago at this point, but I was doing a DOS with some friends on an alt, so it was something sub 10 level -- high enough you should be familiar with the mechanics but low enough that it wasn't a huge deal. We had a hunter in the party who had died like 12 fucking times before we even got to the second boss. No one had said anything to him or complained in party because whatever baby key, but he died again to Haywire or something and just silently dropped group. We ended up four manning the rest without him, and I want to say we even timed it.
I don't even give a fuck if someone in a low key has minimal experience and doesn't remember mechanics real well as long as they say something. If I have to give a one sentence tl;dr description before a boss or hard trash mob, I happily will. I don't take low level keys very seriously and they're supposed to be intro content - it's the place to learn! But no one ever says anything.