What was the ruling on the Littleton TD that was reversed?

What was the ruling on the play that was initially ruled a TD by Littleton? Mahomes' arm was moving forward, but Westbrooks knocked the ball out of his hand before the pass came out. So it seems that the initial call of a live ball was correct. On the other hand, rules change so often (tuck rule, etc.) that it's hard to keep up with them.