I believe it's the profile the links give your site. If most of your links are from a large number of forums across a wide range of forums then it's possible that your backlink profile may resemble that of a forum spammer. When you actively engage forum signature links as a link building method you are setting yourself up to creating a profile that resembles what you have become, which is a forum spammer- particularly if a cursory review of your posts show that your forum participation consists of a few fluff posts on each forum. If your backlinks receive a hand check then it's going to be obvious that they were created with the purpose of influencing the algorithm, which can get you in trouble.
If it's just a lot of links from one or three sites, that's probably going to fall into the range of normal. There would be a lot of collateral damage otherwise.