what would make the A.C compresser frost over when not being used on 1993 GMC Sonoma

it is on a 93 gmc sonoma i am in the the proses of replacing the heater core and was wondering why the A.C compresser was frosted over after driveing but not useing the ac

Asked by for the 1993 GMC Sonoma
If you are using the defrost, then the A/C compressor is still working. Are you using defrost at all?