Since you said "at the beginning of each season", I'm assuming this isn't a one time event. Do your sights go back to normal after a period of time, or do they keep going higher?
My guess would be that you have a form issue, especially, if you haven't been practicing in the off season. Minor changes in anchor, grip (too much or not enough heel pressure), bow arm position, and shoulder/back tension can cause this type of problem and will, usually, cure themselves as you get back into form and recondition your muscles.