I've had the booting back to "xbox home" problem with Forza 5, killer instinct, xbox one fitness and Kinect sports.
I was able to resolve the issue with Xbox Fitness by re-downloading after deleting the file twice. Forza however was not so kind.
Contacted support and we tried putting the system back to default factory settings by going into settings then system and choosing restore factory.
Then restarted the system up. Once back on the main home screen before putting any disc in I disconnected the system from online. (Turn your router off or pull the internet cable out).
Then after it was offline put re-installed forza with the disc and went ahead and started it when it first said I could (I didn't wait for a full 100% install). Problem was gone its working as of now.
I'm going to play it until it's fully installed and then reconnect it online for the update. I'll post on here again after I do. I hope it works for anyone else with this issue and do share.