Is it common no... Does it happen yep.... What causes it, a whole bunch of things... You would have to see it yourself, and often you can tell the cause.
Most often the connection to the buss of the panel gets contaminated prior to installing the breakers and that dust or whatever doesnt allow a good contact surface with the breaker. Essentially a loose connection... Over time it developes an arching condition that generates more and more heat, further decreasing the resistive connenctions contact. That heat starts heating other breakers around it. As metals get hot, they oxidize, and this cycle continues... The bakelite on the breaker starts to deteriorate, and the breaker starts to fall apart.
Likewise the same thing can happen with the contacts inside the breaker itself...
And the same again with the lug of the breaker...
Then there is breaker failure. A high amp load or hard starting motor can degrade the breaker over time. Heating the breaker incrementaly until it eventually does the same thing - heats the bi-metal not enough to trip, but enough to oxidize and enough of that will hold the breaker shut. Then heat it more etc, etc.
A high AIC short on a breaker not rated for it will weld a breaker shut, and damage the breaker enough to have poor contact heat up, etc, etc...
A number of shorts on any breaker will damage the breaker enough to have poor contact heat up, etc, etc...
Salty air, foggy weather, disimular metal contact, poorly made equipment, you name it. Theres a few of us here that have done service work that have seen them all... Once saw one fill with roach eggs and bodies - hold much more than its rating for who knows how long....
I have seen some certain brands of breakers hold 50A on a 20A circuit for a long time and not trip..... (FPE, Zinsco, even SqD) They are not fool proof, if you want that we have to go back to fuses, and they have thier own draw-backs....
But if I were you, I would go inspect the situation yourself..... Check the buss work, and crack the breaker open. If it were the buss work the same thing is going to happen again. If it is pitted from arching or contaminated in any way failure is enevitable.
