I noticed this happened twice. There are situations where suddenly mate-settings-daemon starts chewing up all the cpu cyles when the system becomes idle. It causes the system to become unresponsive, even trying to reboot becomes impossible.
In both situations, I had opened up gedit in a "sudo su" session. it could be occurring in other situations as well.
Killing mate-settings-daemon just respawns it with the identical problem. Logging off and back on doesn't resolve the issue. You need to power off or reboot.