Many corporate travelers can book and re-book with no change fees, so this happens more than you would expect.

The most common reason I've seen is a large corporation cancelling or rescheduling an all-hands / quarterly meeting that everyone above a certain level (e.g. directors / VPs) from non-HQ offices travels to HQ to attend.

For an Oakland to Las Vegas flight, I'd guess a conference, music festival, or sporting event was cancelled.