Automatic updating android apps
Sometimes the new version of a popular app might be buggy, but there's no easy way to revert to the old one, for example.
For more information see Specifying Minimum System API Version.
And then, of course, there are the changes to software that you didn't want in the first place.
Sometimes bugfixes add more bugs, and sometimes new features aren't improvements at all.
By default, Android apps downloaded from the Google Play Store are automatically updated.
If you would rather review the updates and any new or changed permissions associated with the updates, you can choose to disable automatic updates.