I would say it is 99% because of fire crush, I played a lot of games with Ruul since patch and haven’t noticed this. But I do not use that spell.
Cannot test it right now, on account that I have access right now I do not have that spell, so maybe latter today.
I can see a lot of possible scenarios related to that spell, triggering multiple times (like malvolio mentioned in other post sometimes there are multicast), other spells in your hand take that 20% hp loss, cast dragon and crush at a same time and every tick of dragon takes 20% of your HP, minion not killed by it makes you take dmg every time he takes dmg … I could think at least 50 more scenarios. So to narrow it down for developers. You might mention:
-Your device and system
-Time when this happened (is it one time thing or at least 3x)
-What was the deck you used
-What class did you play against (player or practice, if player maybe you can remember who, as he/she might be able to shred some light on it)
-Were there other minions around your hero at a time of death
-Why do you even play with Fire Crush (this last one is for me xD )