I don't think it's because of sports rivalry. It wasn't the mascot who was poisoned--it was other dogs belonging to the breeder of the mascot (which is not where the mascot lives). So probably it's for the same reasons anyone poisons anybody else's dogs. . .because the dogs bark, because the person hates dogs, because these particular dogs look too much like wolves, because some people don't like having a breeder (or anyone else with a lot of pets) in their neighborhood, etc. Some people are just jerks.