For a variety of reasons, there might be more coach passengers than they have coach seats (or biz pax than biz seats). If there are free spots in business they may operationally upgrade (op upgrade) people in coach to the vacant seats in business (or from business to first).

UA typically chooses these people based on their status and can do it ahead of time if the flight is oversold. UA may do it with people that don't have seat assignments over status if something extraordinary happens (such as the cancellation of a flight and all those passengers must be accommodated on a later flight). This is because it involves fewer interactions with passengers.

Based on some of the posts on this board, it seems that op ups from MP members accompany the pulling of upgrades or miles from their accounts. Non MP members of course get it without the withdrawl. Seems kinda unfair.