Quick answer
Moisture has entered the socket or its supply cable. Outdoor sockets must be IP66 rated. Do not use it until an electrician has checked it.
Outdoor sockets are exposed to rain, humidity, and temperature changes — and even weatherproof sockets can fail over time if seals crack or the cover is not properly closed. Moisture inside the socket or its supply cable causes the earth leakage that trips the RCD. Stop using the outdoor socket until it has been checked. When an electrician inspects it, they will check the IP rating (it must be at least IP66 for exposed outdoor locations), the integrity of the cable and connections, and whether the socket needs replacing.
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