I am not sure what this is but this has happened at least twice in my clan. We do Hog Rider attacks frequently and a clan member did an attack, all his Hog Riders were fully healed and all the Hog Riders that actovated the Giant Bomb and were in its damage radius, died. Fully healed level 4 Hog Riders died to a level 3 Giant Bomb. I have to say I watched the reply 3-4 times, it is super weird. And there definitely was no double Giant Bomb.
I'll upload some screenshots tomorrow possibly.

