There are a lot of reasons why a link could be considered bad. For example:
- A link on a page that Google has de-indexed
- A link on a page that contains a virus, malware or a malicious script
- A link on a page from a domain that has a huge negative link velocity
- A link on a page from a domain that has a huge number of backlinks, but minimal traffic
There are many
other reasons, but generally, you want to avoid having your links on pages that are either very weak or are penalized in some way.