Idle Monitor is a totally free program that monitors the working of your screen. If suddenly it detects that there has been no activity for a long time, then it will run a series of actions that you previously configure.
The Idle Monitor user can configure and set the time the monitor should be inactive; after that time the PC can be automatically shut down, a specific application can be run, a webpage opened, etc.
So there is no surprise for you of the PC shutting down suddenly, Idle Monitor shows a countdown timer on-screen before this operation is run.