I get that on my 5s as well. Hope iOS 9 fixes that.It does that randomly, and after the restart it won't ask for the passcode!! It is getting really annoying
Mine has started doing this, I've tried setting up as a new iPhone and it continues to do it. Are Apple likely to swap my iPhone out?
There's another thread about it. These occurrences are called Springboard Crashes and my 6+ exhibits the problem too. It started two or three weeks ago and I now experience them daily.
https://forums.macrumors.com/threads/ios-8-3-random-reboots.1865659/page-5#post-21504641