If you set a key bind to mouse button 9 or more, it switches over by 1 next time you launch the game.

How to reproduce:

If your mouse has more than 8 buttons (which from my research doesn't seem to be common) follow these steps:

1. Set one of your key binds to Button 9 or more.
2. Restart the game.
3. Look at the same key bind only to find it has changed to the next button.
4. Repeat steps 2 and 3 to see it change to the next button every time.

If your mouse has 8 buttons or less, follow these steps:

1. With the game closed, go to your options.txt file and change one of your key binds to key.mouse.9.
2. Start the game.
3. Look at the key bind you changed only to find it says Button 10.
4. Repeat steps 2 and 3 to see it change to the next button every time.