It came from the "cool" kids who rolled horde, but then realised they were the bad guys, so instead did some spin doctering and made out the alliance were bad.
Blizzard saw many people re-rolling horde and in order to keep them happy, changed the lore so that the orcs didnt really invade azeroth, it was the burning legion mind controlling them.
Originally it was humans versus orcs, where orcs were the invaders, all lore and story kinda got added and changed from that point forward.
Basically there is no good and bad anymore, its all got twisted, its just humanoids, versus burning legion, whatever side you choose you will fight for your cause and be at war with the other factions.