What would cause a liquid line to frost in cooling?

Most commonly would be a dirty filter.
Low airflow-check blower speed wiring,belt (if applicable),blower wheel
Low freon

Normally a restriction is going to freeze your liquid line, plugged drier,orfice or maybe bad txv.

A dirty Evaporator coil is the one I dislike the most. Cleaning is a real pain.

Forgot to mention that this is a heatpump, suction presseure goes to zero, suspect restriction,cant see any obvious signs of damage to line set.

Does your suction line also freeze? IF suction is reaching zero or below(pulling a vacuum) you most likely have a restriction(as mentioned before, could be filter drier, orafice, txv). You will also see low or vacuum states on suction side if charge is very low. What does your high side read while low side is at zero? And do you notice the high side dropping also when unit is run for a period?

suction presseure goes to zero

Check your metering device for restriction (orifice) OR if TXV bad power head..

High side reads about 150, this was on a cool day about 70 degrees. Did not run the unit very long and the suction line was not cold.