On Tue, 01 Dec 2015 19:19:01 -0700, Don Y
wrote:

The outlets in the back of the house are on a 20A GFCI.
(Note to self: are there any other outlets on that same
circuit that I need to chase down?)

Presently using them to light XMAS lights on one of the
citrus trees. IIRC, each string is about 250W. With
~700W on the circuit, turning the breaker ON (i.e., using
it as a switch -- instead of plugging/unplugging the
load) causes it to immediately trip. Repeating the
action in short order appears to get it to latch and
remain latched (ON), indefinitely.

OK, so the surge when all the lamps are cold can increase
the inrush current -- on the short term -- to levels that
probably exceed the 20A limit of the breaker.

"In theory"

OTOH, in years past, I've run the circuit at close to its
capacity (~10 strings) without this problem.

I've changed breakers (swapped with one feeding another
circuit) and the problem persists.

[There are no leakage paths in the wiring OUTSIDE]

This suggests something in the wiring/fixtures. I'll
start isolating outlets, tomorrow (dark here, now).
Perhaps some insect (leaf cutter?) has opted to nest
in one of the receptacles.

Anything else I should explore?

Since you are swapping stuff around, try it on a non-GFI circuit. That
will tell you if it is a ground fault.
Usually this tracks back to water/bugs in a box.

I have one nightmare GFI circuit here that is longer than the design
spec for GFIs but it works when everything is dry. When it fails, I
end up splitting the circuit up to isolate the failure. Bear in mind,
it can be a ground fault on the neutral.
On outside boxes, make sure all of the wirenuts are pointed up, near
the top of the box and that they are not too close to the box. It
mitigates the water they all collect eventually.