这个问题可能是由Android系统杀死了应用程序进程造成的。为了保持应用程序的持久性，可以在Application类中实现以下代码来重新启动应用程序：

public class MyApplication extends Application {

    private static MyApplication instance;

    @Override
    public void onCreate() {
        super.onCreate();
        instance = this;
        registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            }

            @Override
            public void onActivityStarted(Activity activity) {
            }

            @Override
            public void onActivityResumed(Activity activity) {
            }

            @Override
            public void onActivityPaused(Activity activity) {
            }

            @Override
            public void onActivityStopped(Activity activity) {
            }

            @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
            }

            @Override
            public void onActivityDestroyed(Activity activity) {
                Intent intent = new Intent(getApplicationContext(), MainActivity.class);
                intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                startActivity(intent);
            }
        });
    }

    public static MyApplication getInstance() {
        return instance;
    }
}

这里的MyApplication类是应用程序的自定义Application类。我们实现了ActivityLifecycleCallbacks接口中的onActivityDestroyed方法，并在该方法中启动了MainActivity。这样就可以实现自动重启应用程序。