Why is propane tank line freezing on gas grill?

This is related to the physics of gases under pressure. The short of it, is that a gas under increasing pressure gains heat, or heats up, and a gas under decreasing pressure loses heat, or cools down. The more rapid the gain or loss of pressure, the higher or lower the temperature goes. You can probably get more technical information by typing "physics of gases" into your search engine, and chosing an appropriate website. The tank has been over filled. When the tank is over-filled, it freezes the regulator up the line. Take it back to where you had it filled.