I haven't seen the play, so don't read too much into this, but if you are in the act of catching the ball, you are considered a defenseless player, has nothing to do with whether or not you leave the ground. Also if a player is in a defenseless position, the hit doesn't have to be to the helmet to be illegal. Lowering your head and hitting with the crown of your helmet to any part of a defenseless player is considered illegal contact.
http://nflcommunications.com/2011/12/27/definition-of-a-defenseless-player/