It's not real hard to figure out.
It occurs when:-
a) A person body pulls and doesn't taunt or generate any more hate within a certain amount of time
b) A mob is inactive (white, unattackable) and becomes active
In case [a] what is happening is:
1) Tank body pulls mobs. Tank has 10 hate (example value). Tank has your group ward on him.
2) Mobs whack tank as he pulls back to the group. Your ward prevents dmg. Mobs now hate you. More their dmg is warded the more hate you gain.
3) Mobs get into aggro range of group and make a periodic hate check. They decide they hate you more than the tank. Banzai!!
In case [b] what is happens:-
1) Mob becomes active. Faced with no hate list and 5 players in aggro range he picks one to aggro semi-randomly (Note: Some scripted mobs are set to aggro certain classes over others. Eg. healers first)
I have had case [a] occur before when using the Bestial Feast line of spells on my Fury. Combat ended. Feast triggered on mobs death. Tank immediately ran off (on 95% hp) and body pulled another nearby target. Didn't taunt during the pull. Feast HoT ticked and restored some of his health. Mobs ran to attack me because I had more hate than the tank. Did not take him long to regain the hate (one group taunt) but it still occured.