The answer to 'why is my GFCI outlet tripping breaker' is that a GFCI (ground fault circuit interrupter) outlet is designed to trip the circuit breaker when it detects a ground fault or electrical imbalance. This is a safety feature to prevent electric shock hazards. Common causes for a GFCI outlet tripping the breaker include a short circuit, faulty wiring, water or moisture exposure, or a problem with the GFCI device itself.
If your GFCI outlet is tripping the breaker, you'll want to first inspect the outlet and surrounding area for any visible signs of damage or water intrusion. Unplug any devices from the GFCI outlet and press the 'reset' button to see if that resolves the issue. If the GFCI continues to trip, you may have a more serious electrical problem that requires the attention of a licensed electrician. They can properly diagnose and repair the root cause to restore safe, reliable power to your GFCI outlet.