I don't know why he was so called, but in 'Nam a sapper was a VC who had the training, ability, and courage to snake through the concertina wire surrounding the post. He was, of course, armed with plastic explosives, and I don't believe he had a long life expectancy.

About this Zulu thing: this is the first I have heard about British origins. We used to sing it as kids, and I just supposed it was invented by kids in some schoolyard.