The answer to 'why is my GFCI outlet tripping breaker' is that there is likely a ground fault or short circuit in the electrical system. GFCI (Ground Fault Circuit Interrupter) outlets are designed to trip the circuit breaker when they detect a ground fault, which can occur due to a problem with the wiring, a faulty appliance, or moisture in the outlet.
Some common causes of GFCI outlets tripping the breaker include: a problem with the GFCI outlet itself, a short circuit in the wiring, a faulty appliance or device plugged into the GFCI outlet, or moisture or water getting into the GFCI outlet. In some cases, the GFCI outlet may be tripping due to a larger issue in the electrical system, such as a ground fault or overloaded circuit.