I've had this problem for several weeks, after 8.1 update I believe, and att replaced the phone and it continued and got more frequent, so they replaced it again, I'm now on my 3rd 1520.
After researching last night I have turned off ALL background running apps in the battery saver, unpinned Cortana from the start screen, and after a restart it still did it about the 2nd or 3rd time I woke the phone up within 30 minutes of a restart. If it continues today I will completely turn off glance screen and restart, but after that I'm not sure what the hell else to try.
This is a pretty serious issue in my opinion, they give us a phone and software to integrate all these notifications and accessories but in order to work properly we have to turn all of it off lol. The att repair center is 45 minutes from my house but if it happens again I will be driving down there and demanding it be fixed or put me into another device, this is ridiculous and I was an early adopter of wp8 with the 920, seems like I am going backwards now with the 1520, and its clearly software related.