Why did my Windows 10 restarted?

Windows Update for Windows 10 was made to look for a time in which a user (like yourself) is not active, and begin updates then.

If needed, Windows 10 will also automatically restart at this time, which is likely what occurred in your case.

Alteatively, the system errored, BSoD'd or lost power and then started back up again afterwards. Given the lack of logs stating as such, I would assume that none of these actually happened.

Also, the log line you're showing here, "The operating system started at system time 2016-07-02T04:51:13.490451600Z.", states that the operating system started at 04:51:13, not 06.51.13.