Is this considered a muck?

[androidMetro]

I am dealing for a home game and not involved in the hand. At the turn Player A checks, Player B bets, Player A declares All-In.

Player B instantly throws his cards face down touching the burn card pile. He hesitates for a second and says Call. I immediately say your cards hit the muck, he pulls back his cards and says that's not the muck pile.

The actual muck pile was on the other side of the table. (I was dealing from the side of the table.) What's the ruling?

For what it's worth, it happened so fast that no one else noticed.