Had this happen for the first time yesterday. Oddly, I thought it might happen before it did. I noticed someone else on this forum had said it seems to happen to them when they leave the phone locked up in an area with no reception. Well, yesterday I had to leave my phone in my car for 6 hours in an area with zero reception of any kind. I decided to leave the phone powered on to "test" this theory out. Sure enough, when I got back in the car I had the "sleep of death" that required a battery pull for the first time. Not to mention the battery was fully charged when I parked the car yet it was down to 60% when I got back in yet it hadn't been used at all. Just a guess, but I am wondering if this might be something it does after searching for an extended period of time with no luck finding a signal.