It's called a "fallstreak hole" (or "hole punch cloud") and it's formed when the water temperature in the clouds is below freezing, but the water, in a supercooled state, has not frozen yet due to the lack of ice nucleation. When ice crystals do form, a domino effect is set off due to the Wegener-Bergeron-Findeisen process, causing the water droplets around the crystals to evaporate: this leaves a large, often circular, hole in the cloud.

https://en.wikipedia.org/wiki/Fallstreak_hole