Breakers trip because of heat.
When heat from wire exceeds breaker rating, it trips.
Something is causing heat on the wire, wire is too small for the amp draw, there is short circuit, or breaker is bad.
Replace breaker and/or test wire.
Put amp-meter on wire and read amp draw through wire.
Amps cause heat. That's why high voltage lines from power station are high-volt-low-amp so there is no heat loss with long distance transmission, until transformer at home converts electricity to lower volts and high working amps with enough power to run electrical devices.
Same is true for AC and DC circuits.
If amp reading on wire exceeds rating on breaker for length of time, then breaker will trip.
The fact that the breaker lets current (amps) to flow for a period of time before tripping is because the breaker is probably rated for slow-acting, or it takes a while for amp draw to heat the wire.
Following image shows amp-meter that clips over wire to get amp reading.
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