Muffins sometimes stick to the paper because of the moisture in the muffin according to the Quaker Oats Company. Baking muffins with moist ingredients, such as shortening or moist add-ins like berries, can increase the likelihood that muffins stick to paper liners.
Luckily, there are a number of simple things that can prevent muffins from sticking to paper liners. Aluminum foil cup liners can be used and sprayed with nonstick cooking spray to prevent muffins from sticking. Muffin tins can be used without any liners and simply greased on the bottom with cooking spray. A special nonstick muffin pan made of silicone or metal that is coated with a special nonstick finish can be used to bake muffins that won't stick to the pan. Following the directions of a recipe exactly can also help ensure that muffins don't turn out to be overly sticky.