Google repeatedly try to trace spam and block or erase them. Google penguine is another tool of google to filter those spam.
Google wants your site to be useful and any links coming to it to be natural. It is not natural to have 90% of the links to your site being anchor text links for your main keyword. These are obviously not natural links in the main, so such sites will be penalised. Reduce anchor text links and have a wider variety of incoming links.
Actually, Google changes it's norms after a period to provide quality results. As some improves their PR using SEO techniques without having quality content. So, Google changes it's policies and it's termed as Google Penguin update.
Actually, google is trying to do his business. Google doesnt mind if by doing it some good sites become banned. Its the collateral damages theory.