Some poor, innocent, hapless newbie wanders into a forum and wonders aloud if that guy in that show should be with that girl in that show. The forum erupts into flames, igniting all the boards that deal with the show, which ignites whole sections of the Internet into a blazing Inferno.
The innocent newbie has opened the door, causing an Internet Backdraft.
It works like this: Controversial Event X occurs, resulting in flame wars, including ones spanning multiple forums. Internet vigilantes occur who muck up the issue even more into a morass of rumors. Eventually it gets to the point where people are so sick of flame wars about Event X, that this in turn causes flame wars conducted by those who want it to just die. Which it eventually does, sort of, in a smoldering-ember sort of way. Then some n00b gives it enough oxygen to make it flare up all over again.
This is also known as "Holy Wars". Trolls and Fan Dumb are drawn to to them like flies to a carcass. Mike Reed's Flame Warriors refers to this, when the metaphorical "door" is opened on purpose, as a Grenade. It is also why we have the Rule of Cautious Editing Judgment, and one of the most frequent reasons for a trope page specifying No Real Life Examples, Please!. Some tropes and/or audience reactions are themselves this - which is why they are listed as Flame Bait.
The single item in common for these is that people outside the involved communities are astounded that anyone could care enough about the issue to get in any kind of argument about it at all, much less a big argument.
It sometimes leads to people saying "Pass the Popcorn."
Not to be confused with Internet Counterattack - That's when the internet acts as one to make someone in real life pay.
See also, Flame War, which is a common terminology for this, Flame Bait, a normal cause for this, and G.I.F.T.. See also this video from CollegeHumor, which sets the whole concept of this to a Billy Joel song.
No examples, please. Everything can cause this to an extent, whether it's on the Internet or not. Besides, having an examples section for this is just asking for vandalism.