Hey Itsjaskrn
Really hard to say anything more unless you can share your site.
But in short, it seems it's not about links to external sites on the pages, it's the pages themselves, so perhaps they're very close to low guest posts, or other things that might be considered using the reputation of your site to rank for things you're not really relevant for.
We have a section on the website for brands to promote
If you mean that brands can add / edit these, it could well be that they were using them to try and leverage this, and kinda fits the bill of something without your (meaning the site, the publisher) editorial oversight.