They happen, and there doesn't seem to be any logical method or reasoning behind them apart from the one constant, if you have status and Y is oversold. But even when Y is not oversold, op-ups do occur, although I do not know why or how.

The best advice is to not expect op-ups and be satisfied with the cabin you booked. And if you do get op-uped, then hooray!

Here's my experience of 3 out of 4 long hauls being op-uped on a trip to the US last year.

Op-ups still happen...