The cause of tumors in rodents is a bit tricky, basically its cancer. Many things can cause cancer, I think a lot of it is genetic. I go through hundreds of rats per month that I raise in my rack system, I've never seen even one with a tumor. I suppose the food can cause it if it's contaminated with a cancer causing agent, but it's very doubtful. I would suspect something in the water too, still probably unlikely.

To me it seems that animals that are very old can develop cancer, almost all of my dogs got cancer when they were near their end of life. Personally if I had tumors in my rodent colony I'd make sure to rotate the breeders and feed off the older ones to keep everyone fairly young. I'd also try a different strain of rat or mouse if you are getting tumors in younger rats, maybe go to a feed store and get a few feeders to raise up and use their babies to replace all your breeders.