Reboots can be caused by a lot of things... not knowing 100% of what is going on, I am just going to go with my gut feeling on this one. One of the most common causes of reboots is a simple kernel panic. The kernel is basically the base code on top of which everything runs... it controls the core hardware of the phone, in particular, the processor. If it runs into something that it can't deal with properly, it'll flip out... 'panic'.... and when that occurs, it really has only one way to deal with the issue; reboot.
They occur from time to time... I've had a couple of phones that had kernel panic issues early on, both times the problem got fixed with an OTA.
I would say try slowly scaling back. Get rid of Nova... if that doesn't fix it, do a factory reset... if that doesn't fix it, return the phone.