Thanks all for your responses; The circulating pump runs at all times, the local thermostat/valve allowing the hot water to route through the rads along the outside walls. The abient air temp may still be high (higher than the normal freeze protection temp) while the rad next to the wall experiences much colder temps and is thus subject to freezing.
These are all individual apartments, with the occupants setting their own temperatures, perhaps even leaving them off in the winter if heat from other units is enough, and/or leaving windows open in the winter (freezing air dropping down over the window sill onto the pipes) setting the stage for burst pipes.
The thermostat needs to compensate for the poor actions of tenants.