shit man, I got no idea...
Only time I have seen something like that in my personal garden, was on my very first dwc grow bout 4 years ago, I put h202 in one of my bubble buckets(correct dose) but didn't mix it in properly, couple hours later my plant looked all droppy and like it wasn't taking up water anymore...
This is simply a guess .. so take with a grain of salt... but i would raise my lights quite, raise my humidity, and possibly put a clear plastic bag over the plants that are dropping, or mist them(making sure no water hits a bulb and blows up)
if a pathogen in the root zone is causing the problem, probably will have to do some type of root zone treatment..
on the plant that looks most wilted, can you snap a photo of this plants roots for us.
Maybe drop the water temps, something I read dds talking about here.
You should give dan a call, ( hope he is free, know they have that huge expo up in denver today, don't know if he goes himself or sends a rep)...