A couple of other things that appear on the replay.
JT is watching intently, and may see something that we can't - maybe the ball coming loose?
It appears that after Clayton rolls over, Bell strips the ball out. You can see that Bell has his arm in there, and is tugging at the ball.
Assuming for a moment that Clayton doesn't have clear possession all the way through the catch, then its possible that the ball *would be* incomplete if it hit the ground; instead it pops up and that's when JT grabs it. So, rather than an incompletion, it becomes an interception (it would not and could not be a fumble, because he would have to have clear possession all the way to the ground - and the ground can not cause a fumble).
I also wanted to point out the linesman who I've circled. He's watching the play and trails it. He's the only one with something close to a clear line of sight. He never rules incompletion, rather he runs with JT to the endzone.
So, in looking this over, I'm thinking that maybe Clayton didn't have clear possession all the way to the ground as is required. He was juggling the catch and never had possession, therefore an interception would be the correct call.