There's a
Tasker task that can be set to do that (if that's the way you want it), but since you don't, I'd say it's either an unintended effect of another app you've installed (restart in Safe Mode* - if it stops doing that it's another app - then you have to figure out which one you installed right before it started happening) or the phone is defective and you should bring it back and ask for a replacement.
*
- Turn the device off.
- Press and hold the Power key past the model name screen appearing on the screen.
- When SAMSUNG appears on the screen, release the Power key.
- Immediately after releasing the Power key, press and hold the Volume down key.
- Continue to hold the Volume down key until the device finishes restarting.
- When Safe mode appears in the bottom left corner of the screen, release the Volume down key.
To leave Safe Mode, just restart the phone.