I think I tried that too and it didn't work for a good reason. From a user standpoint that would be really annoying. As someone is quick swiping between message apps (going left and right) if your app suddenly went expanded, they can't continue browsing. I'm guessing Apple would reject the app if you tried to get around this by using a Timer to launch expanded after a short amount of time.
If you need that extra space (and one of my apps definitely did), your best bet is to just have a button that tells users like... "Get Started" and when pressed it goes full. That app got approved too. So Apple obviously sees the need for going into Expanded mode when the user prompts it.