That happened to me too when I visited the site months ago. A window popped up and asked if I wanted to run a scan as my computer was infected. I clicked on the ‘x’ to close the window without answering and that was the magic password to get lots of nasty things downloaded to my computer.
I took it in to get it fixed and was told that when one of those windows pops up, just shut down the computer without closing anything because a ‘yes’, ‘cancel’, ‘no’ or just trying to close those windows can be programmed to download viruses and such on your computer.
It’s possible you were the victim of a one-two punch, in which a virus sets itself up quietly in your native OS environment then waits for you to do something else specific. Once you do what it’s waiting for, all hell breaks loose. Website content can be programmed to catch an attempt to close a browser window and do something else browsery, like pop up panels or even fill your screen with browser windows, but in principle your browser should act like a sandbox with respect to disk content. The operating system, not the browser content, is designed to be in charge of the interface between disk content and the browser (like save file or upload file).