It can be from cold/ warm cycles of the day too. We're starting to see them more now that the sun is radiating a good amount of heat during the day but at night it still is way under freezing.
The shot was from the infamous winter of 2015, when it seemed to snow forever. I think we got well over 100" that year. (108.6" as of March 15 that year.) The snow in Boston was collected in a big pile on the waterfront and the last of it wasn't melted until July 24. I have yet to break out a shovel this winter. (Fingers crossed.)