What really happens when a resistor breaks down?
What aspect of resistor break down are you referring to? I am going to assume you are talking about voltage.
All resistors have a critical voltage where the electrical current will arc across the resistive material. When this occurs that resistor no longer functions as a resistor or the value of the resistor changes drastically (usually lower) for a short period of time after which it fails and the circuit is now broken or open (infinite resistance).
Niel