iCloud Photostream on work computer randomly

So I have never, ever even physically connected my iPhone 6 Plus to my work PC but I do use our WiFi. I have to do some minor Photoshop work and when I left my desk for maybe 2 minutes and came back, all of my contacts were on the screen. I closed the window and then went back to my photos and all of a sudden, my iCloud is on there and it wouldn't let me delete it (I figured it out). I just don't get how this happened, I've used the same computer & WiFi for years and never had any connection to it from my phone other than through WiFi. The worst part is all of my personal pictures and information was right there for anyone to view (it's used by maybe 10 people) which is why I never synced anything with it. Any ideas, I'm average tech wise but this has got me clueless as I only have my iCloud synced with my iMac.