Offsite “over-optimisation” refers basically to your site’s link profile. There are a few things you can with your links that will look fishy to the engines, so avoid them!
Too many links, too fast – if your link time graph shows a sudden spike, the engines may look closer to check that they’re not spammy or paid. Remember this is relative – other sites might get 1000 links in one day and it won’t look suspicious, but 100 in one day would look strange for you.
Unnatural link profile – anchor text may look unnatural, for example, or your links are reciprocal
Links the content hasn’t earned – Google have quality raters who search different terms and assess how good the sites that are ranking are in relation to the search query. They will eventually pick up on content that is ranking well despite being of low quality or relevance, and may penalize it.