As pretty much everyone has mentioned... RHF door lock has the grounding for the tailgate motor, and I believe it wears the contacts on that microswitch quicker, as they are only rated at 0.1A. I replace them with 5A versions on the latches I rebuild to give a bit more headroom!
Jumping door locks is indeed a sign of the microswitches giving hassle, and best sorted before they fail completely - that leads to additional headaches!
I have a couple of door locks to work on this week for people, and probably have the parts to make one for a 2000 if you are after a refurb unit. Despite being home all the time now as my normal work has dried up with all this virus business, I'm still pretty flat out, so it might not be until next week when I have any available - but usually when I'm away working, it could be months! haha..
Generally if you have jumping locks, then the one that ISN'T jumping with the rest of them is the culprit, as it's the one sending the lock/unlock command to the BECM and it then triggers the other doors.
Regarding why it's doing it with the garage door triggered... RF interference is a fairly big issue on these vehicles - usually in relation to battery drain, but I've had another owner who had a faulty central heating thermostat, I think that caused his vehicle to do all kinds of weird stuff - obviously the actual RF transmission it was sending was enough to freak the BECM out properly. He was one of the guinea pigs for our RF filter and it cured his issue overnight.
I see you're in the Cotswolds... I'm based in North Swindon if you do want a latch - I do them on exchange for the faulty one, and if you're not comfortable fitting it, then I offer a fitting service too.