What can cause that is the contacts inside the breaker are making poor contacts to the metal bars inside the panel. As that starts to melt, then the ones next to it can start to melt and it can travel up the panel. Next thing you know, the whole panel is starting to melt down and you end up with a big fireball where you have to call the fire department instead of an electrician.
This is not something you should ignore, but however; if you had a preventative maintenance program in place, this would’ve caught it and detected it, and you would never have gotten this far.