Why would my fuse box trip if I've already tripped isolation circuit I'm working on?
Was this a neutral wire?  (blue or if older, black) 

The RCD is comparing current flow on "hot" vs current flow on "neutral" (return).  They should be the same.  

You notice on multiple circuits with multiple trips (breakers), only the hot is switched.  

It's sloppy, but sometimes people wire neutrals so they are shared among multiple circuits. Most people consider that a wiring fault.  This doesn't work if each circuit has an individual RCD (like here in the States, and that gets many people converting to GFCI here).  However with a whole-box RCD, it's possible to get away with multiple circuits in that same box sharing a neutral, and the RCD would be none the wiser.  That may be exactly what happened. 

In the USA, because of this problem, we have a rule that neutrals must be pigtailed when (intentionally) shared among multiple circuits.  That way, only a single neutral wire comes to the apparatus, and it can be removed without interrupting the neutral for those other circuits.  UK doesn't use wire-nuts, and the lighting fixtures provide terminal blocks with multiple neutrals, so that passively discourages you from pigtailing.