I think there are lots of potential reasons. I agree that, for me, the heavy sweats seem to SOMETIMES be linked to a hard day's exercise or after a few consecutive days of exercise . My theory is that it is linked to stress in the nervous system (ie the physical stress of the exercise and adapting to it). adaptation happens at night and maybe it is not quite happening as well as it could.
currently I've got this thing (EMFIT QS) that measures HRV (ie sort of stress) throughout the night. nothing obvious yet to report