Do I take that to mean that the one RCD protected everything - including the circuit into which your drill was plugged and the circuit to which the cable you damaged was connected?RCD Incomer.
If so, as I said, I can't see how bernard's hypothesised mechanism could be the explanation, for the reason I gave. However, given that I haven't got any other/better ideas, I'm not at all sure where that leaves us!
Kind Regards, John