Sub pulls inward when it plays.?

I have 2 10" Pioneer pro series subs in my truck powered by an MTX amp. One speaker does as it is suppose to do. When the bass hits the cone comes out. The other sub when the bass hits pulls the sub inward. I checked all the grounds and other connections. I even took the speaker wires and reversed them, good cone reaction speaker wires to the bad one and vis versa. Still the same problem on the same sub. Anyone got an idea as to what maybe the problem?