Likely the reason was due to a momentary torpor. Similar to hibernation but sometimes only lasting as little as a few hours. When it is cold, there are less insects, which mean less food for the bats. The bats know this and choose instead to save energy rather than waste it flying around. Bats have a very high metabolism and consume an enormous amount of food to keep up with it, but are also very good at conserving energy.
Nps.gov goes into this some:
Bats use a lot of energy flying around and must consume a lot of food, such as insects, to fuel their daily activities. When cold weather drives insects away, bats must choose to hunker down and hibernate or migrate to warmer areas with more abundant food supply....
Hibernation involves an extreme reduction in metabolic rate, heart rate, and respiratory rate that allows a bat to survive long periods of time without food. A bat's heart rate drops from 200-300 beats per minute to 10 beats per minute, and it may go minutes without taking a breath. The bat's body temperature can also drop to near freezing, depending on the temperature of the bat's surroundings. Other bodily functions also slow down, which reduces energy costs by about 98%. In this state of "torpor," bats are experts in high energy efficiency!
Bats may enter torpor for just a few hours to save energy during a cold day, or they can remain in torpor for up to a month while
hibernating over winter. During hibernation, bats cycle through
periods of torpor interrupted by brief periods of arousal when their
body temperatures return to normal for a few hours. Some species, such
as this little brown bat, may hibernate for more than six months
waiting for the return of insects in the spring. Bats choose places
like caves, mines, rock crevices, and other structures with ideal
temperature and humidity for hibernation
Full text can be read here: https://www.nps.gov/subjects/bats/hibernate-or-migrate.htm