Huh, I'm glad that I'm not the only one with this issue. My galaxy S5 has started doing this as well... Weird. I have so many apps installed that uninstalling them one by one and waiting a few days to see which is the culprit isn't an efficient solution. As a side note, I had severe battery drain about 2 weeks ago, but the problem has since solved itself. I'm assuming it was a bug in Google Play services but as I said, it's since fixed itself. Anyway, here are some instances in which my phone restarted randomly,
1. After my phone rings from being asleep for a while (screen off), the call will end if I don't answer it, and then a few seconds later I just hear the tmobile jingle and notice that the phone has restarted. Annoying...
2. Randomly after opening some apps. For example, one time I opened Google+, which I don't use often at all, and the app force closed and then my phone froze for a second, and then it rebooted.
3. Sometimes it just does it at the most random times for reasons unbeknownst.
So yeah, there's my issue. I can relate. It probably has something to do with RAM or the memory getting overloaded. The phone probably can't handle it and then just reboots itself. That would be my guess, I really don't know much about this stuff. As a side note, my current RAM status is 1.51 GB in use from 1.81 GB. My cached data is 2.73 GB. That's probably bad, but I really don't want to clear it because most of it is from Play Music, and I don't want to have the app re-download everything anyway. I'm on a TMOBILE galaxy S5. I've had it since it came out. Sad to see this issue arise, thought this phone was 'the one'...