Yu-gi-oh Duel Result That I Don't Understand.

So I'm playing Yu-Gi-Oh! 5Ds: Duel Transfer on the Wii. I was playing against an NPC on the 4th level in story mode that uses a Volcanic Pyro deck. Here's the Run down of the situation:

My LP - 7600
His LP - 6600

He has 2 monsters on his side of the field: Flame Spirit Ignis and another monster with 300 Attack points. I have one monster in defense position. He kills my defense monster with his Ignis, and hits me directly with 300 damage.

My LP - 7300
His LP - 6600

The following turn I draw Injection Fairy Lily and summon it to attack. I attack his monster that has 300 atk points and activate Lily's effect by paying 2000 LP. His monster is hit with 3400 attack and he takes 3100 damage.

My LP - 5300
His LP - 3500

The following turn he summons a Volcanic Scattershot in attack mode. He attacks my Fairy Lily and of course I activate her effect. I lose 2000 LP and he attacks with 500 attack points. His Scattershot is destroyed and he takes 2900 damage.

My LP - 3300
His LP - 600

All fine and dandy right? I thought I had it won. Next turn I would summon a monster, kill his Ignis and attack with my other. But wait...here's the part I don't understand. For some unexplained reason when his Scattershot is destroyed by my Lily I also take 2900 damage just like him. So:

My LP - 400
His LP - 600

His Scattershot's effect then activates from the grave cause it was destroyed and I take 500 more damage and lose the duel.

He attacked with his Scattershot cause the AI knew that would happen if I spent my LP to use Lily's effect, but no where on any of the monsters does it have any effects that would make me get hit with that 2900 damage. I have no idea why I took that damage, and I don't think I ever will. Anyone know? lol