Perhaps this is a test by the forum owners to see if the advertising works. More seriously, it is just random isn't it. They just haven't bothered to filter out non-vegan stuff which, to be fair, would probably be a hassle. I personally don't really notice the adverts. For the most part, my brain just filters out that space.
The forum uses Google Adsense so I somewhat have control over what is displayed but blocking specific ads is a little tricky, I've gone through our ad categories and I've blocked ads that refer meat so hopefully you'll no longer see those ads.
Most of the time Google will display ads based on what you've searched for elsewhere that use Adsense or other Google services, so for example if you're on eBay looking for a book by Terry Pratchett you'll probably see ads on other sites that use Adsense for books by Terry Pratchett. But I have noticed that they're pretty random sometimes too.