Q: Why my iPhone 6  auto power offed during charging or sometime?

Hello, S Linn A.  

Thank you for visiting Apple Support Communities.  

I understand that your iPhone will restart on it's own and during charging.  Here is a troubleshooting article that I would take a look at for this issue.  

If your iOS device restarts, displays the Apple logo, or powers off while you're using it

-Jason H.