Sometimes I think it is because the inside of the boots stink really bad (like mildew).
So when you walk it pushes that air out with every step and you leave scent.

If I can smell mildewed boots in my closet, there's no reason a deer can't smell that.

I wear X-Scent socks and put my boots on at the truck and then spray them down with scent killer.
Seems to work.