Moisture
The most common cause. An outdoor box whose in-use cover was left open, a receptacle behind a leaking sink, a garage box that floods, a crawl-space junction. Water across the terminals leaks a few milliamps to the grounded box, and 5 mA is the limit. Open the downstream boxes (power off) and look for corrosion and damp.
A damaged cable
A nail or screw through a cable, a staple driven too hard, a rodent chew: the insulation is compromised and the hot leaks to the ground conductor or the box. It may trip only in humid weather. An electrician's insulation tester finds it; you can narrow the run by disconnecting the cable at each box in turn.
A shared neutral
If a neutral from another circuit is tied into this one downstream — a handyman's "spare white wire" — the returning current on this GFCI's neutral no longer matches its hot and it trips as soon as the other circuit carries load. Multiwire branch circuits (two hots sharing a neutral) must be fed by a two-pole GFCI breaker for the same reason.
A hidden load
Something is plugged in that you did not count: a hard-wired garage-door opener, a doorbell transformer, an outdoor light with a wet fixture, a well pump. Each is a candidate.
The device itself
Last on the list because it is least likely with a device that resets and then trips again after a while. Disconnect the LOAD conductors: if it now holds indefinitely, the fault is downstream; if it trips with nothing on LOAD, replace the device.
Method
Work from the device outward: LOAD disconnected → holds? Reconnect LOAD, disconnect at the first downstream box → holds? Move one box further each time. The trip returns at the box where the fault lives.