Most likely the pecan pie filling has overflowed the pastry dough while baking and has dripped down the sides of the pan. To keep this from happening try these tricks. 1. Don't overfill the tart shell 2. Lightly grease the muffin pan before adding the tart dough 3. As soon as the tarts come out of the oven run a knife around the edges to loosen the tarts. 4. Use a non-stick mini muffin pan.