The recipe creator intended for the feta to be placed in a ring around the skillet's perimeter to cook separately and achieve crispiness. The user's deviation by filling the center with cheese instead of a ring likely caused the egg to stick. Using light feta, which has less fat, might have contributed to the cheese adhering more strongly to the pan. The heat level could also be a factor, as excessively high heat would further promote sticking, especially with a less fatty cheese.