Bears are opportunistic omnivores. They will eat pretty much anything. Although, most of the meat that they eat is carrion. They can kill, but it isn't common.
It could have died from any number of things. Coyote, fox, old age, disease, bear, etc. There are a ton of things that could have killed it. I'd bet your dog dragged it back to your yard if you let it out without a leash.