Supposedly the result of too much masturbation, according to Anglo-christian mythology since time immemorial. A variant on the "If You Don't Stop Playing With It It'll Drop Off" theory of genito-manipulatory repercussion.
It's all plainly bullshit, however - I've been trying for years, and I haven't had to shave my palms once. I do wear glasses however, so maybe it's just compensating some other way...