In my case, it silently turned on fast start-up again on my laptop and it's not the first time either since upgrading to Windows 10.
I already had an OS crash when Windows 8.1 kept doing this as my laptop is dual-boot with the second OS on a separate SSD (using its ODD bay enclosure.) When fast start-up is enabled, Windows goes into partial hibernation when "Shut down" and in turn keeps files open, including to the file system. So when I boot the OS on the second drive, it sees that something is not right with the other SSD just like an unsafe shutdown, performs a check disk and attempts repairs. So when I later boot the primary SSD, it does not know that the file system was altered, particularly with handles to files and the file system that were still open in the hibernated state and can lead to a rather nasty mess the next time the OS is booted.
It's now come to the point where each time I need to boot into the other OS, I have to first check that "Fast start-up" is disabled and the same again in Windows 10 on the second SSD which also intermittently turns that setting back on. :doh: