After a lot of investigating I found out how to reproduce what happened to me last night:
LEAVEMALONE = Hunter Pet
Censored Name = My Holy Paladin
This is what happened:
I used Divine Sacrifice in 2on2 Rated ( EU ) while playing with my hunter mate. During the duration of Divine Sacrifice, the opponents killed the hunters pet, which caused me to take like 88k damage ( obviously insta-gibbed me )
I can not confirm, that this also works with Warlock-Pets or Deathknight-Pets. Could be useful in 3s though, since there are a lot more Hunter/Paladin/x Comps.
Enjoy while it lasts.