Just happened this Monday.
Four of us flew IAD/AMS on a cheap, cheap sale winter fare for the weekend on UA. None of us had elite status, although all 4 of us had Mileage Plus numbers. On check in for the return, they informed us that we would be traveling in business class for "operational reasons." Obviously, they had oversold coach and undersold business class, but why they picked us to upgrade is beyond me. The only thing I can think of is (as suggested in response to my question on the UA forum) that the flight was full of non-MP members on discount tickets, and we looked marginally better because we all had some previous MP activity.
BTW, though I was pleased with UA's business class service, the seats were not so hot -- the non-extendible leg rest is pretty uncomfortable. A lot better than steerage, though.
Charlie