Skype update text contains next:
App auto restarts if unexpectedly shut down
How is possible to perform that via SDK?
As far as I know, some sort of apps can be run in background and can be restarted in specific case. This is from Apple docs
If you start this service and your application is subsequently terminated, the system automatically relaunches the application into the background if a new event arrives. In such a case, the options dictionary passed to the application:didFinishLaunchingWithOptions: method of your application delegate contains the key UIApplicationLaunchOptionsLocationKey to indicate that your application was launched because of a location event.
My opinion is: Skype is relaunched with remote notifications mechanism.
Update
Well, I think I found something. @Malek_Jundi was half-right. Your app should fail with non-zero exit code, but it should be registered as VoIP application.
Because VoIP applications need to stay running in order to receive incoming calls, the system automatically relaunches the application if it exits with a nonzero exit code. (This could happen in cases where there is memory pressure and your application is terminated as a result.) However, terminating the application also releases all of its sockets, including the one used to maintain the VoIP service connection. Therefore, when the application is launched, it always needs to create its sockets from scratch.