I've talked about this with several other economists a few times over the past couple years. It's not something we have a total handle on, but here are some thoughts on what's going on:

1. Many people retired during the pandemic.
2. Those retirements opened positions which pulled people out of the lower tiers of the economy.
3. Many immigrants returned home during the lock downs, which greatly increased low wage openings.
4. Many women had to drop out of the labor force to take care of their kids during school closures and some number persisted with homeschooling.
5. The Quiet Quitting phenomenon has people using more of their leave, so even filled positions are less reliable than normal.

The big yet unanswered question is why wages haven't adjusted to fill so many of these positions, but it might just be that a lot of these jobs don't make sense at the wages required to fill them.