Same here on my QNAP TS-210 SINCE(!!!) Update to 3.4.0 Build 0212T!
The reboot problem happened 3-4 times since update to 3.4.0, never happened before!
In my case, the problem has nothing to do with iSCSI (service is disabled) but it happens unexpected! Also when there is no access to the device, for example in the night, sometimes like today (see log) also during the day with absolut no access to the QNAP at the time of reboot!
Disks: 2x Seagate ST3500412AS CC35 (RAID 0)
280 10:12:00 System 127.0.0.1 localhost [Strip Disk Volume: Drive 1 2] The file system is not clean. It is suggested that you run "check disk".
279 10:11:40 System 127.0.0.1 localhost System started.
278 10:11:40 System 127.0.0.1 localhost The system was not shut down properly last time.