There are a few reasons why bacon might curl up when cooking.  1) One possibility is that the bacon is not cut evenly, which can cause it to cook unevenly and curl up.  2) Another possibility is that the heat is too high, causing the bacon to cook too quickly and resulting in curled-up bacon.  3) Finally, it could be that the bacon is not fresh, which can also lead to curling. If you're unsure why your bacon is curly, it's best to ask your butcher or check the package for more information.