If he had an SWU requested, I suspect that that will clear before an op-up. Op-up's are usually a last resort to get all the pax on the flight they are scheduled to be on, by moving extra pax in Y to C when there are no seats available in Y. UA would obviously prefer to recieve something for that. Hence, they would go for taking an SWU (or buy-ups if they can get them) vs. giving a C seat away for free. This is what makes the most business sense.