QB fumble?

[phansen]

NFHS

QB A1 is attempting to pass the ball, but ends up scrambling. B2 dives at A1's feet and causes A1 to stumble forward. A1 stumbles a minimum of 3 full steps albeit off balance before placing the football on the ground in order to regain his balance. A1's hand slides off the football which he placed on the ground. The ball stays exactly where A1 put it. A1 then fall to the ground about 3 yards ahead of the ball, reaches back and tries to recover the ball.
IS this a fumble? Or was it down when the ball touched the ground since the ground can't cause the fumble. How long is B2's contact a factor?
Rulebook citation appreciated.