He saw the Roughing and got a stiffy, and started running back on the field. Thought the play was over with I'm assuming. He noticed the returner coming up the sideline and 360'd it back to the sideline. What made it worse is that he had his helmet off. If his helmet was on, they might not have even noticed.
Who knows, that civilan everybody was getting on to might have yelled for offense to get back on the field, and Brig. took off running.