Achievement Hunter is a YouTube gaming video/let's play collective that operates as a branch of Rooster Teeth, a machinima studio most famous for the Halo-based Red vs. Blue series. With the popularity of an established brand backing them, affable personalities and a sense of humor that's at least a cut above the average YouTube LPer, their sizable fan-base comes as no surprise.
The actions those fans take, however...well.
As with any group of fans this large, many of them have sunk into disturbing behavior patterns: drawing horrible porn and cringey fanart of Minecraft LPs, shipping infant versions of Real Human Beings together (no, really), and generally being fucking terrible. It has to be stated that all of this is over people who play video games on the internet. But that's not even the worst of it.
The Achievement Hunter fan-base comes mostly from tumblr, and is therefore completely mired in tumblr culture. This means one thing primarily: a vast majority of them base their entire lives around people playing video games, and are completely unable to accept that a bunch of gamer dudes might not care about social justice politics as much as they do. Case in point:
Generally after an incident like this happens, people will do anything from threatening violence towards AH themselves, to sending death threats and harassment to fans who support them, to harassing their spouses for retribution and whatever else is considered a reasonable reaction to someone making a joke within a 50 mile radius of tumblr.
This isn't the first time something like this has happened in the fandom, and it definitely won't be the last. There's a lot more to all of this, more blogs, more horrible fanart, more jokes that caused fan shitstorms, but I'd rather cut it off here before it gets too long. I think this is more than enough for anyone seeing it for the first time.