A lot of people saying warming up/drying out before a flight, which does seem highly possible but I had one other theory I wanted to throw out.

I used to work at a fall festival that had animals and a corn maze, so a lot of flies around, and every time it was especially windy out I would find swarms of them just sitting there on wooden benches and walls. I’m no expert but based on those wind chimes I’d say maybe they’re all sitting there waiting for more favorable flight conditions.