Some apps causing buttons to not respond and then crashes

Some apps cause the power and home button to not respond. The app itself continues to work but I can't return to home screen. After leaving it for a while, it goes to apple screen and then back to lock screen.

So far it has happened on two "fullscreen type" game apps (the sandbox, tapped out).

Does anybody know what is happening? Is there anything wrong with my phone?

iPhone 5s, iOS 7.0.4