As far as it turning on by itself, the phone doesn't weigh enough to depress the button. More likely, something is pressing on the phone enough so that the power button is depressed. IE: Cat walking on the phone, using the phone as a coaster, book placed on it, etc.
This phone is designed for you to lay it face down, not back down. Of course you don't want to do that without a screen protector and/or a good case. It's been said that proper storage of all SmartPhones is face down. This way if something drops on the device it will have less damage. Samsung has even taken it a step further with their new Galaxy 6 Edge. If it's face down, you can program the sides to display different colors for texts, emails, and phone calls. Of course this only works face down.
I personally like the buttons being on the back. They are more ergonomically designed and fit my hand better. In fact, I tried the Droid Turbo for a few months and found that I missed the back buttons. However, there are some who hate them. It does take some getting use to.
Good luck.