Nobody's sure on what the motivation behind it is, but I can't think of any other reason someone would have any desire to do this.
They are the parents and the siblings of that particular mascot, not just any tamaskans.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.