Sorry for your loss :/

Redleg is a fast working illness, but not that fast. It sounds like a bacterial infection that has been there for a long time, or like something strong (chemicals, agressive disease) has attacked him/them.

With such a seemingly sudden and weird death, I would trust it has inflicted the others as well. I encourage you to fill out the form Strider18 suggested and - if it's possible in your area - take them to the vet, maybe get fecal samples done, just get them checked.