I mostly did my testing using a portable TV in the loft, firstly plugged
into a spare output, and then having unplugged all the other outputs.
Didn't seem to make much difference. I don't think I tried it in every one
of the outputs, though. What causes shorting? (Just occurred to me that as
my aerial is in the loft, a storm seems an unlikely reason??)

Shorting could be caused by nailing through a cable, or a crap connector at
one end (or a fault in a TV/VCR Etc.)

If you tried a few, with all other outputs disconnected, it sounds like the
unit has failed for whatever reason.

Do you have a mast head amplifier too (another box of tricks, usually right
next to the Ariel)?

Sparks...