for MOST people it has to do with over-arching dominance. If you're right-handed, chances are you'll spin left, and vice-versa for lefties. That's why more people spin left naturally -- there are more right-hand dominant people out there.
There are always exceptions, but normally that's how it flows. Switch hits are a bit up in the air. That has more to do with the shoulder you look over naturally. Say you are right handed but skate goofy: chances are you learned switch right before switch left.