They just replayed the ref's explanation on ESPN, he didn't say anything about the end zone. He said that as the receiver's knee came down (which was kind of garbly and sounded like he might have said something about the end zone) the ball came loose thus no reception and no touchdown.
What I heard them say was the issue was that the receiver was falling down as he was catching the ball. Any time that that's the case, the receiver has to keep control of the ball when he is ruled down (by contact with the ground in this case).
I could have sworn he said end zone as I heard it.
old news now though.