A strange thing happen sometime, Jenkins start displaying " Jenkins is going to shut down" even when nobody turned this message on and restarting Jenkins.
Screenshot:
This message is displayed when someone with enough administration rights has clicked "Prepare for Shutdown" from the global configuration, or went to http://jenkins/safeRestart
URL.
If you are absolutely sure that none of the administration have clicked "Prepare for Shutdown" (which you cannot be sure of, unless you are the only administrator), then you should look for someone using your credentials, either directly in browser, or through command line.
Review your webserver's access logs and see if anyone or anything (some script) is accessing the /safeRestart
URL and /quietDown
URL.