Motion, shift or nothing

[schwinn]

We had this happen the other night and I think I got it right as the R. A is at the line. QB says set and everyone except A2 goes from a two point stance to a three point stance. About a second after they shift and a half a second before the ball is snapped A2 goes down to his three point stance and his hand hits the ground right about the time the ball is snapped. I throw the flag and call an illegal shift. I'm about half convinced this is right. Is this the right call?