At the risk of being told that I don't get it, I have seen the pocket close in.
I believe it occurs when both the OL and DL "fail", much like in M12.
I've seen when the the CPU QB is still looking for somewhere to throw and the RE has pretty much reached the QB, but still engaged with the LT. Sometimes, this cause him to throw away, sometimes he'll look for other space in the pocket, stepping up and sometimes either running or somehow throwing beyond the line of scrimmage for the voiceless illegal forward pass call.
Since it happens very infrequently, I'm thinking it's the "double fail" scenario, much like in M12 where you didn't get the patty cake, 'stalemate' at the LoS, or the shove him down/quick 'engagement'. Otherwise, it's the usual stuff on both sides - being locked down for ages or under pressure in almost no time.