Do maggots come from eggs?

Today I caught a regular house fly and I put in a container and into the freezer. I wanted to see if I could freeze it and then bring it back to life, which I did. However, something weird happened. As the fly was thawing out I noticed a small maggot exiting its rear end. I watched one maggot after another squirm out of the little hole in the rear end - about 25 maggots. I thought maggots are laid as eggs? Not born as a maggot.