A lot of labor jobs do just that and shut down and lay off their guys through the winter. Only the more important stuff continues, so if it's still happening, there is likely a good reason for it. A lot of them are currently trying to bang out the last bits of work they can before the Temps start dropping below freezing regularly, at which point even more of these companies call off work.

Hard to answer why without knowing the details of specific sites and the work they're doing.