Anxiety alone is perfectly capable of causing twitching. Anxiety causes adrenalin to go shooting through your body; over time, your nerves essentially lock into the fight or flight mode that they're used to experiencing. So they're way more sensitive and way more susceptible to imbalances in your electrolytes, increased anxiety, etc.
Your body is all out of whack when you're anxious. It unbalances so many things, and your nerves are one of its favorite subjects. Think about all the extra chemicals you're producing with your anxiety - adrenalin, cortisol, etc. It's no wonder our muscles sometimes feel a bit out of control.