I played Spirit Peddler, a 6/6 Rush Deathrattle and attacked a 4/4 Taunt.
The Spirit Peddler should have had 2 health remaining, but died instead; triggering it’s deathrattle.
What was the blocking minion? Could it have had venom/poison?
Plenty of reasons this might have happened.
Maybe the other minion had poisonous.
Maybe there was an effect that increases damage dealt to minions.
Maybe you’re mistaken and the other minion had more than 4 attack.
Etc etc etc.
No way to tell for sure, without a game replay recording.