Like other animals, butterflies have a temperature range in which they can survive. Having laid their eggs on members of the pea/bean family such as clover, their eggs have hatched by now and their over wintering caterpillars are under cover for the winter. These butterflies are apparently following their biological imperative out of habit, perhaps for one last fling.
Research described by EurekaAlert describes the double jeopardy facing some butterfly species with tolerance to a very narrow range of temperatures. They are threatened by climate change in their native range as well as changes in their habitat. Fragmentation of habitat limits their ability to seek new territory and constricts their gene pool.
Fortunately, our ubiquitous clouded sulfurs are oblivious to both threats and the calendar. With more hard freezes ahead, I just hope their kids are safely tucked in bed for the winter.