Place the pork shoulder in the slow cooker.
In a bowl, mix together the barbecue sauce, apple cider vinegar, brown sugar, mustard, Worcestershire sauce, chili powder, chopped onion, minced garlic, salt, and pepper.
Pour the mixture over the pork shoulder.
Cover and cook on low for 8 hours or until the pork is tender and shreds easily.
Remove the pork from the slow cooker and shred with two forks.
Return the shredded pork to the slow cooker and mix with the juices.
Serve on buns with additional barbecue sauce, if desired.