iMac (mid 2010) High Sierra - Not Shutting Down

When I shut down my computer via the dropdown menu in the my computer does not always shut down . Instead I'll come back to find my computer re-started and a message on the display saying my computer was shut down because of an error.

I don't understand why it's restarting after the shut down. When I return to my computer to discover this scenario, my computer fan is running and my computer is very warm.

Any suggestions as to what the problem may be or better yet what the solution to the problem may be?