Place the chicken breasts in a slow cooker.
In a bowl, mix together the pineapple juice, barbecue sauce, garlic powder, onion powder, black pepper, and salt. Pour this mixture over the chicken.
Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is cooked through and tender.
Once cooked, shred the chicken using two forks and stir it into the sauce.
Toast the hamburger buns lightly if desired.
Assemble the sandwiches by placing a generous portion of the pineapple chicken on the bottom half of each bun. Top with sliced red onion and pickles if using, then cover with the top half of the bun.
Garnish with fresh cilantro if desired and serve immediately.