Two schools of thought on the rut:

The same time each year - the impetus being the shortening of the days/photoperiod.

-or-

The moon phase in addition to the photoperiod. If I remember correctly, right at the beginning of the new moon. I could be wrong on that.

I tend to subscribe to the moon theory as it seems to be matching up in my area this year. I just started seeing seek and chase behavior in the past 3 days. Give me 15-20 more years and I will let ya know...