In regards to fractals, I'm of the thinking that this is largely attributed to agony and the weighted nature of the boss fractal. Often, the posts that crop up about being kicked at the end of a run is from fractals (less frequently from dungeons). I believe this is largely attributed to how poorly the game explains what agony is and why it is important to get agony resistance for fractals. I'd not be surprised if the authors of these "I got kicked from the end of a run, lets all grab some pitchforks" posts are frequently under-geared and generally unprepared for fractals.

I don't think that it is a coincidence that many of these posts say "I was kicked at the last boss" where it becomes very quickly apparent whether or not you have agony resistance. Most players aren't going to watch how much damage you take from agony until you get to the final boss and take unavoidable agony damage before the fight begins. I truly don't blame someone for seeing a party member lose 80% of their hp (or just outright die) and thinking "Clearly this party mate of mine is unprepared for this, and will be more of a hazard to the party than a contributor".

Seriously, is it fair to those four people to do the content with 20% of the group dead before the fight begins because one person doesn't understand that they're under prepared? If the game were to make it very undeniably clear how agony works and why agony resist is important, I feel more people would be less ignorant of that mechanic.

To expand on my second point, again I don't find it a coincidence that the boss fractal is where people are kicked more frequently because of the (often) spike in difficulty. Assuming even moderately hard rolls, you're often not going to find difficulty in the first 3 fractals like you get in the final fractal.

Most groups roll for swamp and either afk underwater kill mossman (don't get me started, seriously) or get bloomhunger and fall asleep while he dies. Worst case scenario after that is getting cliffside, which most groups end up running around ranging archdiviner (again, a whole other topic) or bugging him out, followed by the grawl fractal. Every other fractal is pretty moderately easy, but once you get to the boss, things can get ugly fast (even if everyone has appropriate agony resist).

Aside from Jade Maw, the boss fractal is designed to give a significant increase in difficulty. This is where having more coordination and experience is supposed to come into play. If you find one of your members is largely inexperienced or under-prepared, that's the first person to get the axe. I'm all for people teaching beginners and for helping out their fellow man, but at the same time I entirely understand why someone would be frustrated to find that someone joined their group without properly preparing themselves.

So TL;DR: Get your agony resistance, familiarize yourself with the boss mechanics, practice, push yourself to be a better player, and you won't end up making "I got kicked from a fractal" posts.