Is that the replay official felt the Rebel defender was responsible for the Auburn receiver going out-of-bounds. If that was the case, as long as he came down with the ball in bounds, then it is a completed catch. It probably took so long since it was difficult to tell whether or not the <span style="text-decoration: line-through;">right foot hit in bounds</span> left foot was still touching the out of bounds line when he touched the ball. If it was, out of bounds. If it wasn't, completion.
He couldn't definitively say for sure, so the ruling stands.
(Edited to correct ruling.)