I think looking at this, we can see the defender is looking back at the QB and the throw (as he was the whole play )as pickens is just turning his head and still yet doesn't see the ball...I think the defender realized the flight of the ball and dropped back into the endzone, as I think he was covering for two potential plays, one the cut from pickens to front of the endzone and the other the fade. defender here in this image has the leverage over WR, as if the throw was to pickens, he could cut in front of him or drop into coverage for the fade. .