I read this this morning; an hour later and I found my phone off...
iPhone 6s Plus 64GB
iOS 9.1 v3
Restored from iPhone 6 iOS 8.X backup
iPhone 6s Plus 64GB
iOS 9.1 v3
Restored from iPhone 6 iOS 8.X backup
Update #1: iOS 9.0.2 is released.
Update #2: Issue confirmed in isolated cases at Apple forums
View the latest data here.
We are getting a lot of data now which takes time to put together. I've forwarded the document, which is being updated daily, to Apple's engineering team.
Issue:
Many users have found out that their iPhone 6S, 6S+ (some 6) turns off by itself, even when left unattended. It is fixed by forcing a restart of holding the home and power button for 10 seconds. Had this issue? Read below.
iOS public release users:
Please post your model, OS, and actions before the problem occurred.
iOS beta users:
Please post the same info as above, but also report it through Apple's bug report.
Sample Diagnostics:
iPhone 6S 64 GB Gold
iOS 9.01
1. Plugged iPhone 6S to USB on Windows PC
2. Changed region to US where iOS restarted itself automatically
3. I didn't see the Apple News app so I restarted manually with the power button
4. I checked a few minutes later and the home button nor power button could wake the device
5. I restarted the device holding the home and power button for 10 seconds
https://support.apple.com/en-us/HT201559