These are photos of the inside of my main breaker. We had some power issues and have replaced the panel, which is 35 years old. This is a 200 amp breaker.
What could have caused this failure? Simply old age?
That was sitting there going "BZZT BZZT" throwing bright and shiny arcs across those contacts. You were lucky - this usually is much worse.
Generally those contacts live in a "hard closed" position where the mating faces are mashed together and there is good continuity between them. No arcing happens in those conditions. So what would make it start arcing?
I'm not sure. I don't autopsy enough breakers to know what to make of mild pitting on the other pole. Interrupting high current is very demanding on a breaker. Is this breaker often used to manually interrupt current? Is the house often overloaded to the point where the main breaker trips? Outside of a major appliance failure or wiring bungle, the main breaker should never trip - if it ever does in normal usage, that is a sign of an overloaded panel. A NEC Article 220 Load Calculation should be done on the house, and if it's overloaded come on back and talk to us - we have options to sidestep a service upgrade.
Of late we are seeing many overloaded panels because people buy EVs and believe level-2 charging must be 50-60A (incorrect) and toss it on a panel without doing a NEC Article 220 Load Calculation.
Overloading can cause arcing by simply overheating the conductors inside the breaker. As they heat, they bend somewhat, and they also take some of the spring out of actuating springs. As such you don't have as much force holding the surfaces together.