Mint is known to have laxative effects in large doses. While you don't swallow chewing gum; it's possible that just chewing it releases the flavouring which is swallowed when it mixes with your saliva. I wouldn't worry about it- you would probably have to eat near to a whole packet in one go to do it, but the same applies for mints too.
I've read that things with spearmint are more potent to the whole laxative thing.. but the best (or worst depending on your viewpoint) natural (natural as in not intended to be a laxative) source is dark chocolate.. too much of THAT and you'll regret it