It was "Illegal batting". One of the Steelers batted the ball forward in his attempt to bat it out of bounds. That penalty gave GB a first down (which is ridiculous, but the rule). The problem with it is moments before the illegal bat, the Steelers picked up the ball and lateraled it, which should have established possession and made the bat legal. But the refs claimed the Steelers never established possession. Since that's not reviewable, the play stood. However there's an argument that Tomlin could have challenged that the guy that tried to lateral was down before the lateral, and that if he'd challenged that aspect, they could have kept the ball. But Tomlin didn't do that.