Place pork butt in the bottom of a slow cooker and turn heat to High.
Whisk teriyaki sauce, vinegar, pineapple juice, garlic, pepper, and red pepper flakes together in a bowl. Pour sauce over the pork. Cook on High for 9 hours.
Transfer pork to a large baking pan and let sit for 20 minutes. Shred the pork using 2 forks. Ladle in some of the cooking liquid if pork is too dry.