It's because deep down inside everyone there's something that burns, most of us given the level of annonymity, that the internet does, feel less resistance to the urge to cut loose some of it.
Take it or leave it, humans are a warlike and agressive breed, and when it comes down to it, the stronger a person's hold on any particular opinion, the more agressive they will get in any case where that opinion is questioned, especially when they know that they're pretty safe from the threat of having their bones broken, or life taken in the defense or attack of said opinions.
I generally find ignorance irritating, although I mostly start debates or little arguments with the intention of opening other people's minds or helping someone else grow, and learn about a wider spectrum of this greatly expansive world, also it helps me learn and grow to see other people's opinions and to strengthen my debating skills.
Sometimes I post things to straight up help someone out in a situation I've been through hundreds of times and thusly have a pretty good grasp on the intricacies therein... Seems a fair number of people fail to see my posts as helpful, hell some have even seen them as inflammatory, though no offense was implied, fortunately the mods here are human and not always unwilling to forgive accidental missteps, and trodden toes on my part, I speak like a padded hammer sometimes... Though if I have caused undue harm to someone's feelings, or have spoken out of turn I apologize and rectify the missunderstanding as directly and quickly as I can.

I also have this knack of text walling even when I'm trying to be brief... I know it's annoying, I'm trying to curb that.
/end of wall