This paper confirms the finding (google search: droplet transmission "winter"). They're uncertain of the mechanistic cause, though. They summarize:
> Cold and/or dry conditions impact the stability of influenza virus particles, the innate defense of host nasal epithelia, and the production of infectious bioaerosols. Each of these effects contributes a plausible explanation for the impact of [relative humidity] and temperature on respiratory droplet transmission, and more than one mechanism most likely contributes to the observed transmission outcomes.