Why do some bumble bees sleep on plants at night and not return to the nest?

My daughter and I observed some bumble bees remaining on a plant after sunset and want to know why they did not return to the nest.Why do some bumble bees sleep on plants at night and not return to the nest?
It may be that the air had gotten too cool to allow the bee to fly. Bumblebees have small wings to allow them to crawl into narrow spaces. To fly, they have to beat their wings very rapidly. This requires a lot of energy and their wing muscles can't generate enough energy to fly if the air is cool. Late in the season, the colony generates male bees known as drones. They leave the hive and search for young queens to mate with. After mating, the males die. Perhaps some of the bees seen were males at the end of their lives. Workers will also leave the nest just before dying.