it is pretty much clear that it is because of temperature, yet what specific effects lower temp plays is still a matter of research, and i find this interesting wrt. various studies of epigenetics emerging in recent times:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4142806/

" we hypothesize that, at least for some loci, the sex-specific epigenetic remodeling process might be influenced by the temperatures at which male and female gonads are exposed during the acquisition of a new imprinting. "