ground can't cause a fumble. He had possession, hit the ground and the impact caused it to pop out.
He needs to make a football move before he has possession. He couldn't do that.
He did. He jumped up, caught it in the air, twisted his body, stumbled back and hit the ground.
None of which constitutes making a football move after the catch. If he hits the ground, he needs to hang onto the ball; if he stays upright, he needs to make a move to run with the ball.