Question: When orientation changes in landscape mode, start a new activity. After back to this activity when orientation in portrait mode in Android.

Answer: 
Yes this the default behavior of any activity. If you want to stop this you can  

- Set the activity `asandroid:configChanges="orientation"` 
 Then Override the onConfigurationChanged like this 

```
@Override
    public void onConfigurationChanged(Configuration newConfig) {
      super.onConfigurationChanged(newConfig);

    }

```

If you want to react to the changes refer this  

[Faster Screen Orientation Change](http://developer.android.com/resources/articles/faster-screen-orientation-change.html),  

[Retaining an Object During a Configuration Change](http://developer.android.com/guide/topics/resources/runtime-changes.html#RetainingAnObject)
I think that you can set only one orientation in your app so that you cannot change orientation like written in this [Q&A](https://stackoverflow.com/questions/582185/android-disable-landscape-mode)