It's probably because those apps haven't implemented state restoration properly. Basically, the OS reserves the right to terminate your app at any point when it

It's probably because those apps haven't implemented state restoration properly. Basically, the OS reserves the right to terminate your app at any point when it is in the background, and it is the job of the app to restore the state back to where it was. iOS provides hooks and a serialisation mechanism for the app, to make it easier.