I had a rat that did that. I don't know if it was because I did used to feed her through the cage and she thought I was food or if she couldn't see very well and didn't know what I was.Eventually as she got older she stopped doing this,but I don't know what caused her to do bite anyone that got close to the cage.She even bit my dog's nose once and made it bleed.Maybe you can ask your vet what might be his problem.
Ratties Rock My World!
Spring,Lynard,Georgia,Teddy,7 of Minnie's children, 2 dwarf rats and our rabbit Charlie
*Proud Rat Owner Since 2005*
Raven,Gracie,Chloe,Roxanne,Brandy,August, Nicki, Lulu,Lainey,Minnie,Edward,Jasper
RIP Lance We Still Miss You