Kroma wrote:
I don't know about an earth rod. Is that likely in a 15 year old house?

So this would happen even if the socket I was working on is unpowered?

Thankfully it didn't happen with the other 4 - that would have really
unnerved me!

You touched the neutral. Neutrals are dead common.
You were, at that time, an earth. What went out
did not equal what came back. The RCD noticed this,
and went "CLACK!". I have had it happen,and if your
head is near the switch, it sounds quite exciting.