Lunging toward the endzone and falling in the direction of the endzone aren't the same thing. Had his trajectory changed like it would if you pushed off a foot, then it'd have been a catch. He fell after bobbling the ball at the top of the catch and securing it on the way down, but didn't maintain possession to the ground. Incomplete, and I don't hate the rule, because he bobbled it and eventually lost it when the initial propulsion of his jump took him to the ground.