How to fix iPhone 6s Shutting Down On Its Own? This Might Fix It

iPhone shutting down on its own?

It’s very possible one of the many apps you have installed may be causing these random shut downs. http://techaeris.com/2015/10/21/is-your-new-iphone-shutting-down-on-its-own-this-might-fix-it/

What the Genius bar says

How about if it keeps getting in a boot loop

Wait for the update from Apple or do factory restore and then restore everything from a latest iCloud backup