Lewis you fucking weegie pussy, you're sweating because you suck.
Seriously though, it probably is just sweat, though <U>could</U> be a leak, iirc some people put duct tape on the inside of their shells around the bottom cable I think, though I haven't done this and haven't had a huge problem. If the outside of the liner is dry then it's obviously not this.
Either way it's not a huge issue, just put some newspaper in the boots after skiing and they'll be dry.