A lot of things: the high rpm motor powering it, the combined area of the blade edge that is higher than the area of the wing edge, and the higher pressure diff

A lot of things: the high rpm motor powering it, the combined area of the blade edge that is higher than the area of the wing edge, and the higher pressure differential between the air behind the propeller compared to the air in front of it. The sound results from this pressure differential collapsing so anything that increases the pressure differential will create more sound.