There have been several interesting discussions on this topic in the TR Tips section including: http://forum.hunting.net/asppg/tm.asp?m=358222 He makes the distiction between the peak BREEDING time and the peak BUGLING time, which aren't neccessarily the same. I suspect that the peak breeding times in your area don't vary that much from year to year. The differences in the peak bugling time (what I suspect you are calling the peak of the rut) is harder to explain, and I supect is due to lots of factors, some of which are: WEATHER (probably most important), increasing elk populations, variations is hunting pressure, etc.