What's the most common cause of A/C refrigerant lines freezing?
You may be looking at the problem backwards, the freezing lines could be a symptom of the problem not the cause.  Start by looking for blockages in the system that would cause lower air flow / heat exchange.


Dirty filters.
Clogged ducts.
Closed dampers.
Closed/Blocked vent covers.
Dirty coils.


Basically if the system cannot exchange the heat/cold, it will not function properly and could lead to over heating/cooling.

Frozen lines could also be caused by high or low levels of refrigerant, so you'll want to get a service person out to check the refrigerant levels. 

Here is a really good image to visualize how it works.