Interestingly, there is a population of spiders in the lower socket. The light switch and the outdoor plugs were both installed at a later date. The switch is in a screened in porch and rarely gets wet. Not so the outdoor sockets.
What the GFCI does : it compares to a very precise degree the flow of current through the neutral and the hot AC wire. The 2 must equal. If there’s a tiny leakage of current from the hot AC wire - possibly a dirty socket is enough, or dirty terminals inside the socket itself, to something other than the neutral (such as the grounded case in the socket), that creates a ground fault and trips it.
Or, the circuits performing the measurement are very sensitive and can age, causing false trips because the circuitry is detecting a current difference when there isn’t one.