Husain Sumra, writing for MacRumors:

Since the iPhone 6s and 6s Plus have been available on September 25, many users have begun reporting 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.

This appears to be an iOS 9 issue, rather than a 6s, 6s Plus issue.

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.

This is happening (I’ve confirmed this via Twitter) on both the latest released version of iOS 9 (iOS 9.0.2) as well as on the latest beta (iOS 9.1 beta 3).

No response from Apple yet.