An old video of mine has just been pushed as an advert according to this. Its an interview about copyright from years ago. I've seen this happen to a few others recently too, is this a glitch or has someone decided to sponsor a video?

That's exactly what happened. Someone ran your video as a Google ad. So, here's a YouTube quirk. There actually IS a feature that allows you to stop people from doing that but it ISN'T included in YT Studio by default. (Though, check... sometimes you get lucky). YT Studio > Settings > Channel > Advanced Settings > "Only Linked Google Ads accounts can use my videos as ads". If you don't have that feature, reach out through the "Chat with creator support" feature in YT Studio, tell them your issue and explain that you'd like that feature activated for your account. If you run into trouble, ping me.

So anyone can run an ad on a video even if they don’t have control over that account?

No. To be clear: Not run an ad on a video, run your video as an ad, basically putting paid reach behind it the same way the creator can. You'll sometimes see companies do this if a creator made a video about their product (sponsored or unsponsored) and they liked it.