Bandwagon resentment, at some point it became mainstream to hate anyone who has achieved more than you have in any outlet. Pro sports, gaming, financial stability, health, etc.
Person A has more than person B therefore person A must be an unpleasant person.
Since the end of wrath and the start of cata but more so with the introduction of LFR at the end of cata, I have watched the server population of my server becoming increasingly more hostile to raiders and the thing is, the raiders on my server are not running around asking to be praised or worshiped, they simply just post a guild recruitment post to find a new member for a raid team and the flames come on.
It is like watching an extremist feminist rally, there is not rhyme, reason, or method to what they do other than I suppose they are aiming to find some sort of self validation in attacking the raid population of the server.