This is a software issue and the issue has been around since early iOS9 and then gone after the iOS10. If your phone is shutting off unexpected way then,
The solution to that is update the operating system to the latest version.
It is recommended to keep the good health of your battery too, to do that always do a full charge cycle each week. You should use the iPhone until its battery indicator shows that the battery charge level is below 20%, then charge the iPhone fully.
If this is happening after the upgrade then seek some help from Apple. You could do full reset etc too to see if those helps but I doubt those will help.
Update (Nov 21 2016)
There is an iPhone 6s Program for Unexpected Shutdown Issues. The program covers iPhone 6s devices within a limited serial number range that were manufactured between September and October 2015.
Apple advises that, if you have experienced the issue, you should visit an Apple Retail Store or an Apple Authorized Service Provider to have your iPhone examined and the serial number checked to confirm your eligibility for a battery replacement, free of charge.
More information and instructions can be found here.