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Some iPhone 6s Owners Seeing Their Phones Randomly Shut Off
Users on Reddit, the Apple support communities and the MacRumors 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.
Users experiencing the issue can contribute to the data pool in this MacRumors' forum thread.