If you cooked the pork for too long on too high of a heat, it can become tough and rubbery. If cooking the pulled pork on high, you need to start checking it at the 4-hour mark. There's much less room for error when cooking on low in a Crockpot! Your pulled pork also might be tough if you forgot to cook the pork fat-side up. The fat melts as the pork cooks, which essentially bastes the meat in its own juices and keeps it super moist.