One cause is a cognitive bias called projection bias. Essentially living inside your own head your entire life makes it exceedingly difficult to understand how others do not also live your same life, think your same thoughts, and hold your same beliefs. In many cases it's quite frustrating to try to empathize and understand why you yourself may not be the center of the universe, which generally results in one 'acting out' in various ways.

So, in short: the internet.