A silent push notification launches an app in the background to update content. Once a user force quits an app iOS will not launch it from a silent push notification until the user opens it again.
UIApplicationDelegate documentation for
In addition, if you enabled the remote notifications background mode, the system launches your app (or wakes it from the suspended state) and puts it in the background state when a remote notification arrives. However, the system does not automatically launch your app if the user has force-quit it. In that situation, the user must relaunch your app or restart the device before the system attempts to launch your app automatically again.