Thumbs closes on its own.

I have a muscle spasm that comes in both hands not at the same time specifically in my thumb's cushy part or where the meaty part below the actual thumb is, the thumb moves toward the center of the palm it usually happens when I am tense, I can move it back using my other hand and then it will go away, I am a computer animator and spend most of my time excessively using the keyboard but why is only my thumbs affected they are the least use fingers, is there a name for this, there is no pain while this happens, just when I am trying to pull it back either with the same thumb or with help from another hand.

Thank You.