It's too simplistic to talk about too much pressure or too little pressure - it's related to pressure ratio (exit/inlet) and the mass flow. Turning on the heater is opening a bleed - as has been stated - which then modifies the pressure ratio and increases the surge margin. Some aircraft have a switch on the collective that is used to force open the normal bleed valve, even though the Ng is higher that the value at which it normally closes, if it is anticipated that the aircraft will enter conditions conducive to disrupted airflow - typically turbulent airflow on landing or flying through hot gas emissions. Ice is definitely nothing to do with it although it's true that ice can cause surge by disrupting the local airflow.