My theory is a combination of (3a) (you have two 3's :) and an additional hypothesis that you didn't mention: people have a pandemic-induced sense of pent up an

My theory is a combination of (3a) (you have two 3's :) and an additional hypothesis that you didn't mention: people have a pandemic-induced sense of pent up angst and unease with their life, and so they are making life changes in an attempt to find happiness again. Obviously, changing jobs won't relieve the pain the pandemic has caused, but I'd argue there's a subliminal desire to try to change something. Anecdotally, my personal experience supports (3a), since I tried to find a new job in mid 2020 and found the job market incredibly challenging, but then looked again in mid 2021 and ended up finding a job I was excited about.