Just thinking out loud here, typing really.
Everyone's nerves run in a general location but not exactly where an anatomy book says they should be. It could be that your wiring or anyone else's wiring could be positioned off a little compounded by the motorcycle riding. It sounds strange I'm sure. I know that I have CPS pretty bad in both my hands. While doing NCTs and the like the doctors found that the nerves coming from my neck, down my arms are not positioned where they should be, compounding the problem. It turns out that, while doing normal stuff, like sleeping, raising my arms up over my head etc. has been increasing the speed of the nerve damage.
So, after all that, I would imagine that some people are more predisposed to a condition such as drop foot from riding a motorcycle and others aren't. Once again, just typing in cyberspace...