Said reasons are typically due to another site/artist actually knowing about the site enough to complain due to loss of sales/copyright/ownership, or just a basic respect for said content creator (i.e. patreon content only, western paycomics typically, or anything that is "owned" by Fakku). Which is fair and understandable, let's be honest.
For example, off the top of my head western wise, someone posted a bunch of Jollyjack's comics that you have to pay for and that's considered forbidden content. Stuff by a certain artist doing a Ben10 comic where the characters are aged to the first season will be fine to pop up as long as no one posts exclusive stuff not on their 'free' site. Fakku has a notable hentai presence and is one of the few English distributors so they get the rights to a lot of releases stateside and as such anything that is "rightfully theirs" will be tagged/outright removed soon after (for the most part... some stuff does slip through the cracks). Basically, if someone doesn't want their work here, it gets the forbidden tag.
Obviously if you're not on sadpanda then other stuff is forbidden, but it isn't even supposed to be tagged as such/should just be removed due to restrictions not on sadpanda (i.e. something with incest isn't "forbidden content" tag wise, but don't expect it to stay up on e-hentai while it'd appear as normal on sadpanda).