Breaker box or heat pump problem?
You should contact an Electrician or HVAC technician.  There seems to be a short-circuit somewhere in the circuit supplying the heating unit. DO NOT reset the breaker again until the problem is fixed.

The reason I suspect a short-circuit, is because the breakers are reacting so quickly.  If this was an overcurrent situation, the main breaker would likely not trip (unless the panel is severely overloaded).