if you any changes in on page like update content, meta tag, change designing than your ranking could be drop down.
Thank you for your advice..My site dropped too when i add some unique content, just like farphp5.. Dropped to unknown rank from page 1..I think you're absolutely right about this..
Content relevancy is a factor for increasing the rankings. but never for decreasing rankings. No matter how irrelevant your post to the niche of your site, that won't effect anything for bad.
Keep in mind that newer sites tend to fluctuate a little more than trusted established sites. I do not know if your site is new or not, but if it is you could see your site bounce around for a little while until the search engines gain some trust in it.
Did the new content have the same subject as the rest of the content on the website? Maybe google is thinking your site is spam?
It might be because of Google Panda, just like what the others are suggesting. It is not just only because of duplicate content but it is also the quality of the new content that you posted. So, the change of your search engine ranking depended on whether or not you produced high quality content for your five different sites.
Dear post always fresh content and do not use links farm in any post it may be cause of black hat SEO.
Adding content doesn't have to do with rankings. But as my girlfriend says, you'll be back stronger if you drop. Stay away from black hat/selling links and you'll be just fine.
It could be that you were undergoing the "Google dance" give it a few weeks and if nothing changes, revert back to the way it was
adding unique content will never make your ranking drop, it might due to other reason such as your competitor are doing better on SERP
Usually there is no reason for a drop unless you are getting penalized. But I don't think that's your case.