Why does the low side A C line freeze?

More than likely a blockage in the evapotartor drain that come out of the bottom of the inside a/c unit in the car. The hose gets clogged up with stuff. If you can get underneath there and stick something stiff but soft(won't puncture anything) in it, if water come running out, you have probably fixed your problem. If that doesn't do it, a low charge could cause it also but I doubt it.