There was a discussion on the penalty given the Tampa Bay safety, Mark Barron, when he picked upRG3 and slammed him.
NFL Rulebook (http://www.nfl.com/rulebook) It would appear that the game officials can apply the rule from : Rule 12, Section 2, Article 7
Article 7: Players in a Defenseless Posture.
(3) a runner already in the grasp and whose forward progress has been stopped;
Bookmark the NFL Rulebook for future reference. There is a lot to absorb. No wonder the substitute officials had problems.