I also have this issue, I also use an nVidia GPU for transcoding (750 Ti). Identified Plex as the issue after changing the time scheduled tasks start and the reboot time followed. Since I've been tracking it, it happened on 4/9, 4/14, 4/28, and 5/1, so pretty sporadic. I have Plex log to my Syslog and attached the logs around the most recent 3 incidents. According to the logs it looks like thumbnails, but there are plenty of times thumbnails are generated without the crash. I'm guessing it's something else that doesn't log.
plex reboot logs.txt