The best guess is that it was parasites, and the girls are just taking a while to get over them. The second guess is that it is parasites and my dewormer is ineffective for either resistance or under-dosing*. The third guess is some sort of virus that they are all mostly recovered from.
The oddity is that none of the babies were scouring. Of course, I've been generous in my overestimations for their deworming doses, because I don't want to have another Starbright or lamb-in-a-ditch episode like last year.
*Interesting enough, Dr. Vandry recommended I try giving the Dectomax orally at 1 CC per 30 lbs, not per 50 lbs. So it could be just that I was under-worming them.