any chance you capture the UART log just before the reboot?
also, we recently also switched the kernel to 4.9, is that the build you are using? with 4.9 we are seeing one issue that leads to reboot , which typically has
[ 5851.372321] Unhandled fault: synchronous external abort (0x96000010) at 0xffff00000a16a8ac
in the UART log
we are looking into it..