The only times that my phone has restarted spontaneously I was running the stock launcher. Since running Nova Prie, it has never happened with that as the default launcher. After the 4.4.4 update I made the stock launcher the default again, since the double home bug was fixed - and the phone restarted itself again. It hasn't happened again since I went back to Nova (it also hasn't happened on Aviate while I've been testing that.)
It's quite possible that the launcher has nothing to do with it, but I'm staying away from stock from now on. (The other problem with the stock launcher is that it will spontaneously stop trusted bluetooth from working. At some point, with the phone on, the phone will go back to the lock screen, waiting for a PIN, even though I still have a notification that my phone is unlocked. This continues until I toggle trusted BT off and back on again. This doesn't happen with Nova.)