Im pretty sure its a reflex..

Never happened to me

refelx

I would assume that it is related to convulsions (Uncontrolled muscle contractions). If you have ever seen a girl go into a full body orgasm, then you know she basically loses all motor control; since muscles tend to contract (causing us to tense up) when we are stimulated (whether it be because we are tickled, or because we are having an orgasm):D we clench our muscles; thus, why toes curl.;)

It's pretty intuitive.

You're muscles are going to contract pretty powerfully. This includes the muscles in your toes. Only, when they contract, they only have one way to go.

release of tension...feels good...but tiring afterwards!

Feels good to contract your muscle when masturbating, not just the toes.

my toes always curl when i masturbate

You do it voluntary, on purpose?
I can't control that.