In a blender, puree the chiles, adobo sauce, pineapple juice, maple syrup, tomato paste, 2 tsp. salt and 1 tsp. pepper until smooth. Pour the chile mixture into a large, lined slow cooker and add the beef. Turn the beef a few times to coat, then arrange fat side up. Cover and cook on high heat until fork-tender, about 6 hours.
Using a knife, scrape the fat off the surface of the beef; transfer the beef to a work surface. Using two forks, shred the beef, discarding any excess fat. In a large bowl, toss the beef with 2 cups of the cooking liquid. let stand until most of the juices are absorbed, about 10 minutes, then season with salt and pepper. Serve as a main course with side dishes..