For a flight to have gone from 120% to nearly empty would mean that some large group didn't show up. Maybe an entire tour bus was delayed by a flat tire or something. Statistically speaking, a hundred people simultaneously missing their flight is vanishingly unlikely.
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.
Statistics generally (there are exceptions of course) makes very broad assumptions about independence and random selection of your samples.
For a group of people as correlated as having the same destination at exactly the same time using the same airplane I would be very wary of applying any statistics at all, even ignoring the possibility of a large group.
Hundreds of otherwise independent people can decide to not show up simultaneously due to terrorist attacks, corona, a cheaper alternative, a conference being cancelled, etc.
