My hot take is that it’s a few factors.
* pent up demand from last year with people not moving out of fear but now feeling certain of the future
* remote work opening up a larger selection of companies to work at.
* tech companies having record profits and all wanting to expand at the same time.
And I feel like #3 is the most important one. They all want mid-senior developers but every other company wants them too so they go in to a bidding war and their wallets are stuffed right now so the bids are huge.
This will probably continue until a new wave of developers reaches senior level but by then the market may have failed to predict tech companies growing at an even faster rate than they are now.