So, I'm not sure if this is a bug or working as intended, so I'm putting it here. I discovered this while attempting to do Crate Runs while riding on a skateboard. They won't talk to you while you're on it, which, is fair enough. However, you apparently cannot mount a skateboard while your hands are occupied. Why? It's not a bike, you step on and push off.
If it's a feature, then it seems like it'd be a good idea to consider allowing folks to be able to mount a skateboard while holding anything in both hands. It just makes logical sense.