What causes an expansion valve to freeze?
The TXV should never freeze up during normal operation. Ice buildup on the TXV usually indicates that the TXV is stuck closed.
Heavy ice buildup on TXV.
The thermostatic expansion valve regulates the flow of refrigerant to cause a pressure and temperature drop in the refrigerant. Heavy ice buildup at the TXV is not normal and usually indicates that the refrigerant is changing state at that point rather in the evaporator.