That often happens when the muffins are overbaked. My old oven ran hot so I adjusted recipe temperatures by 10 degrees or started checking if muffins are done 3-4 minutes early. You test if they are done by poking the center of the muffin with a toothpick (or wooden skewer). If it comes out clean the muffins are done. This test over-rules how long the muffins should stay in the oven : ).