DIAGNOSIS That's the 'Surge Mode' feature kicking in, but it's probably being triggered by a sensor glitch. The built-in particle sensor is reacting as if there's a sudden spike in pollutants, telling the fan to ramp up.
FIX The sensor lens is likely dirty. Turn off and unplug the unit. On the back, you'll see a small, dark plastic window—that's the sensor. Gently wipe it clean with a dry, soft microfiber cloth. Avoid any liquids or cleaners. Plug it back in. Also, make sure the unit isn't placed right next to a source of intermittent airflow or particles, like a heating vent, a doorway, or where someone frequently sprays aerosols.
IF THAT DIDN'T WORK If cleaning doesn't help, perform a full power reset. Unplug it for 5 full minutes to let all capacitors discharge. This can clear a temporary sensor calibration error. Plug it back in and see if the behavior continues.
HEADS UP Surge Mode is a designed behavior, so it turning on high isn't inherently a fault. But if it's happening constantly in a clean, stable room, it points to the sensor. There's no way to permanently disable Surge Mode in the settings.