How does rudder lock occur?
Normally, the airflow over the vertical stabilizer is from front to back, which means it hits the front face of the rudder, and acts to centre it. But the vertical stabilizer is actually like a small wing. In a sideslip, the airflow comes more from the side. If you're slipping tail-right, using left rudder, the airflow comes from the right. When the vertical stabilizer stalls, the airflow is no longer acting to centre the rudder: it's just pushing against it, and pushing it away from the centre.