The game saves your world every time you open a command block's GUI menu. Which I think it isn't intended for that to be happening. Assuming it's intended for the game to save when you open the pause menu, but saving when you open a command block's GUI is quite odd; leaving me to consider it being a bug.
NOTE: Before you read steps to reproduce,
make sure you have launcher set to
"Keep the launcher open" so you can see the
output of the game. If your confused with any of these
written steps; there will be a video for visual learners
below after steps to reproduce.
Steps to reproduce:
1. get a command block by doing:
/give @p command_block
2. Open the command block.
3. (Assuming you have the launcher opened).
You should see the output that the game saved
your world name, the end, and the nether.
This might effect only single player since in multiplayer
you don't pause the game when opening a command block's
GUI.
Need a easier want to understand this?
Luckily for you there is a video!:
https://www.youtube.com/watch?v=CibsIYxwNSw
NOTE2: Before you make comments
or possibly say this is a duplicate, please consider
trying to confirm this bug and seeing the video.
Not just simply comment and move on.
This is intentional. It's been this way since command blocks were added to the game. There was a bug during the 1.8 snapshots where it didn't pause the game, which was fixed because it's meant to pause: MC-68140