alt Hacker NewsFans have a minimum speed needed to spin up when cold starting from 0 RPM. So it's safer to start at near max RPM and spin down to the desired target RPM if the controller doesn't know the exact operating characteristics of each fan.
You can observe this with a speed control knob/ESP32 where a running fan will happily run forever with the knob turned down to e.g. a 30% duty cycle. But after turning to 0% and waiting for the blades to fully stop, if set back to the same 30% they won't move at all or you'll maybe hear a sketchy clicking/whine sound with the blades intermittently twitching.
I've always assumed it was to calibrate the PWM sensor for fan speed, so it starts with no sensing active, runs up to max, and then the sensor kicks in and settles it down. You don't need to run a fan at max speed to get it to start, most will start with very little power applied.