Depends on the context and also user interactions. Top tags will be different depending on the board you are looking at. Users are free to tag any post and the top tags would collect all the tags that are most used. If someone has gone into a number of old topics and tagged the same thing then it may likely float it to the top of the tag list.
If you think someone is playing about with it/abusing the function, report the post and a mod will remove the tags if they don't add anything to the conversation (like website names to spam, insults and unrelated comments). We have removed some tags before as I can get a list of all tags used across the entire site and get particular ones deleted if needed.