Why does my roof heater consistently trip the circuit breaker?
The breaker stays on before you switch it on, so the fault is in the heater.

These things can be damaged in a number of ways - squirrels may snack on them, ice may drag them around and cut them on a gutter, old age and sunshine may damage the insulation. I suspect the only reliable cure will be replacement, but you could (after snowmageddon 2017 is over - or what we used to call a nice normal snowstorm) look for any obvious faults (with the breaker off, please) if you are comfortable getting up to the roof, when you can do so safely.