Have you been consistently posting content? I have seen Google penalize sites who do not update their site in favor of sites with regularly updated blogs.
Google has also been issuing more penalties for onsite web-spam techniques. If you are trying to hide text/links on the page this could be throwing up a red flag to Google. The amount of links you have on your page is also more important now than in previous years. If you have a lot of links on your page, it may have dropped during a recent update.
Obviously if your competitors earn some high profile links and you do not have a solid backlink profile, this could also be a reason.
Start off by looking into the onpage optimization and backlink profile of the webpages that are now outranking your site. Sometimes you drop because someone else has a good SEO strategy.