Well most times it is the tank's fault, so I guess people are just too used to going after the tank.

If tank pulls more mobs than what the healer can handle it's the tank's fault, if tank is losing hate too often while DPS are hitting the right mobs it is his fault as well.
DPS are at fault if hate is loss while they are not following marks or if adds are not killed fast enough for DPS checks (Haukke HM and Pharos first boss)
Healer... Maybe overhealing?