If the main breaker gets too hot, it will blow. Using all those motors on a 130 pound without batteries robot most likely just over stressed it. Was the practice longer than a normal match? The length might have made the breaker get too hot, blowing it.
Some teams that use 6 motor drive trains take a can of compressed air turn it upside down and blow over the breaker to cool it down between matches. This seems to help with preventing blows.
Depending on how hot it gets, either can happen. If it fried I recommend checking the size of your battery cables. Are they up to spec? Normally the breaker would trip before frying. I recommend spraying it with the air just to be safe. ALSO, if you accelerate and then decelerate very quickly, it can be a big stress on the breaker. If you need code to slightly slow down your deceleration, you can pm me for some pseudo-code.
Leav,
The breakers are temperature sensitive devices. If the wire was loose on the either or both of the terminals or the wire part of the terminal, the temperature would rise causing the entire breaker to be warm to the touch. It is possible that is all it was. When the breaker is warm it is likely to trip at a lower current.