Potholes

I have some questions about potholes. The roads were fine, and then it got cold for a couple of weeks, and suddenly there were all these potholes. Big deep ones too. But why does this happen, and so quickly? And why to only certain spots in the road? And why always in a round pothole shape? Its like circles of road just disappear from underneath the thin layer of asphalt or something.

It was way colder for months and months of the year in Banff, I don't recall this happening. You would think the streets would be one big hole as a result. Maybe they had a dedicated pothole fixing team for the winter months or something.