This is because of antivirus software on those computers. It's the same practice complained about here:
Many of the antivirus applications intercept HTTPS connections in order to scan the content of the session. To do this, they add their own root CA to the system or browser and then act as a man-in-the-middle or intercepting proxy between the browser and the web server. On such systems, you would probably see every site's certificate appear to have been issued by the antivirus vendor, unless they have explicitly whitelisted a handful of large or "trusted" sites to allow direct connections to those.
I gave a preview of some documentation I'm working on, with common reasons for this error, at
Maybe one of these reasons applies to your situation?