Doubtful it is the breaker. Most likely there's a wiring fault in the box that contains the switch or possibly where the light fixture is.

To rule out the breaker, I'd move the connection to another in the panel.

With the breaker off, (open, tripped, etc.) take a multi-meter and read resistance between the two prongs in the socket. Should be infinite. Just as second check, take a resistance reading between the "hot" socket prong - the thinner, "non polarized" one and the ground terminal (the round-ish, D-shaped one) - should be infinite. If not infinite in these readings then you have a direct hot wire to ground short. Since you're there, a resistance reading between the "polarized" socket prong and the ground socket prong short be minimal (like, ideally zero ohms). Check all other outlets for signs of water/bugs/nests/contamination by popping the outlet cover off for a visual inspection.

Defective breakers that trip for no reason are rare.

Worst case scenario here: the person who buried the cable didn't screen the backfill and a rock has damaged the cable.

If you disconnect everything at both ends and your ohmmeter reads something less than infinity across any two wires in the cable, it's a bum cable.

Morning all. Thanks for the advice and comments. Issue is resolved. I found a broken ground in the circuit in one of the outlets. It was a good opportunity to inspect everything, since it's outside. Took it all apart, then put each leg back in and tested one at a time.

That's great! Nice job in being thorough with your inspections. It's worth the effort to check things over, even if nothing wrong is found, at least peace of mind is!