If the breaker was tripping immediately, I would think a short. However since you say it runs for 12 minutes, I think a reasonable explanation would be one of two things.
If your well pump is clogged or obstructed the motor will work much harder to spin, thus it would draw more current and create more heat. This could explain the situation. Second guess, and easier to check. Poor connection. If electrical wires get loose from the contacts they will generate more heat (which is current draw). Check the wiring connection at the pressure switch, and for grins, check the connections inside the breaker box as well. Be sure to be aware of electrical safety on that second guess,..
I am not a water professional, just a homeowner.