it happened to my 928 once before getting the last 8.1 update, the signal was so bad and it was heating really bad and it crashed and went black and rebooted, once it rebooted i saw the battery at 13% or something, then i rebooted it again and back to normal, maybe yours are getting rebooted without you knowing and you see that bug, it happened with my old nokia 5230 when using EDGE and maybe with my N97 too, seems like nokia phones hate bad cellular conections xd