Why does this website download a video to my PC as soon as I visit?

[Roger]

There is a website that always instantly downloads a video to my PC as soon as I visit it and it is really annoying because I have to go to download manager and cancel every time I go to the website. It doesn't even ask or anything, it just downloads it. Any one know why? And are there laws against that sort of thing or because i am aware of the download, would there not be any? Thanks for any helpful answers.
The website is for my storage company (although I don't really trust them) so I have to go to it for paying bills etc. and I do have security software. I was pretty sure it wasn't illegal or anything, I was just curious... One can never know to much. Thank you for answering.