Insulation doesn't 'make' heat, it only slows the transfer from the hotter area to the colder area. If it was that cold, I'd expect that zone would have been calling for heat, and it shouldn't have happened. But, if you shut it off, or turn it down at night, it could. If the line runs close to the ceiling and there's insulation above it, as long as the room below was heated, it should not happen. Many heat tapes should not be installed in insulation...you'd have to read the instructions carefully to see if it was an allowed application.