A spammy backlink profile, usually left behind by the previous agency
This is the most common one. The site used to rank, then the rankings started sliding, and the owner has no idea why. The content is perfectly fine. But when you go through the backlink profile there are dozens or hundreds of spammy links, bought from sites whose only purpose is selling domain authority.
Under the current Google updates, that kind of profile pulls your rankings down. Read our take on understanding Google penalties.