Add the salt, pepper, cayenne pepper and garlic powder to a small bowl and mix. Rub pork shoulder with spice mix.
Place the pork shoulder in the slow cooker, add the onion and garlic and 1/2 cup of water.
Pour 1/2 cup of spicy BBQ sauce over the pork and cook on low for 8 hours (or high for 4-5 hours).
Once cooked, remove pork shoulder and shred. Discard the fat.
Add about ½ cup to 1 cup of spicy barbecue sauce and mix.
Toast the slider buns under the broiler or on the grill until lightly browned. (optional)
Assemble sliders.