I've seen it happen, but not too frequently. There may have been an issue, but just don't know. I do know that the TSA 'set up shop' just prior to the gate, and they had a system where they chose certain numbers of people to move to their line, prior to boarding. Bags and ID's were rechecked. It was like every 5th person or such.
I would imagine that if there is no 'issue', it serves to keep the honest honest.