I can't tell you what exactly causes this but this is a Common kind of infection disease caterpillars get. Usually all of them die when some get it. I think they become Infected very fast but then it Takes a few days for symptoms to occur. The reason for this is probably a Lack of Hygiene. Maybe they get the disease from the hostplant. However already sick caterpillars can't be saved. Unfortunately the best Thing you can do is seperate all the caterpillars and hope some of them are not infected. And you should be very careful to not bring liquids etc. From one Box to another. The liguid is a big Source of infection. I wish your caterpillars the best.
Best regards,
David