Why did my RCD trip when I was working on my de-energised lighting ring?
Shorting neutral to earth can be sufficient to trip a RCD, so there's a good chance that's what happened.

It's unlikely that the breaker or switch is on the neutral - though it is good practice to check that there is no live supply to the fitting before working on it, even with the breaker and switch turned off just in case it is fed from a different circuit.