I do this, too, with my left foot. There is no reason why. I just started noticing that I am doing it again when riding... I had a problem with this when I was pregnant with my first son, 27 years ago! I thought it was some weird pregnancy pain, when my foot was hurting like hell from it, and mentioned it to my ob-gyn. We figured out I was clenching my toe every time I pushed the clutch in my car! Henceforth, this disease was called "clutch toe."
DUH.