I asked a few of my goat-keeping friends and, whilst they didn't come up with anything conclusive, they did share a few ideas.
First a link to a blog post about
Recent goat miscarriage: possible causes & implications
And a few thoughts from a very experienced UK breeder
"Things come to mind for scour/weight loss of young kids - clostridial infections, e-coli, salmonella (would have a high temperature and tend to die suddenly) coccidia (no high temperature and looks like plain old ill thrift, they go pot-bellied and thin as well as having dark diarrhea).
Older kids, worms is the most obvious, clostridia again. Clostridial vaccines may not work as well on goats. Some deficiences, like B12 and selenium can give similar symptoms but don't usually cause death."