Usually because they need to return the ball to the officials and communicate with someone on the team about the next play at the same time. In the case of the gif, probably because it was a good catch and it looks badass, too.
http://www.seahawks.com/videos-photos/videos/Golden-Tate-80-yard-touchdown/5ada0db0-6730-407f-bf5a-1db94db4cc1e yeah. His expression at the end is priceless, though.