Definitely environmental.

I'd like to mention that giving a seed mix to a rat who has never had it before easily can make it hack and joke on the splinters resulting in death. Not fun.
Mice can give respiratory infection to rats. ASFs I don't know about. The amoinia from the urine builds up nastily in tanks. When rats come into new environments they already are pretty vulnerable, I suggest doing a quarantine of all new rats and housing them somewhere else for now.