This actually happened to me... Same route (SLC -> BOS). They were looking for 9+ volunteers on that flight.

At first I thought they were just way oversold... But then they started pulling bags from the hold. My parents flew home, I volunteered. They said my seat in C+ was left empty, along with several others. So I can only suspect "must fly" cargo, or way to many skis on board.

(Ironically, my luggage was *not* pulled even though I stayed an extra day and many other bags were. Darn priority tags :-P)