Since the iPhone 6s and 6s Plus have been available on September 25, many users have begun reportin that their 6s or 6s Plus will randomly turn itself off completely, even when it's left unattended and with sufficient battery power. It's unknown what causes the shut off, but some users have also reported their home button feels warm to the touch when trying to revive their phones.

Users on Reddit, the Apple support communities  and the 3uTools forums have experienced the issue. While 6s and 6s Plus users have seen the majority of incidents, some iPhone 6 users have also reported the issue, indicating that iOS 9 may the root of the problem. 

After the issue turns off the phone, the only way to turn the phone back on is to hold the power and home button down for 10 seconds, forcing the phone to restart. MacRumors' forum users have been organizing around the issue, creating a poll to see how many people are affected and gathering data (model number, OS and actions before shut down) to send to Apple's engineering team. 

Thus far, the data collected indicates that no specific action causes the shutdown. Additionally, iPhone 6s users and iOS 9.0.1 users appear to be the most affected by the bug. 

Source:MacRumor