- your website or blog has been scraped of content (something that afflicts a lot of artists whose content is stolen)
- the site that scraped your content now ranks higher than your site in Google (for the relevant keywords)
There's obviously some new initiative to improve the quality of responses to search queries and the algorithm which ranks websites and blogs. My guess is they're seeking to eliminate or de-index the worst offenders!
Another good way of tackling those who scrape content is to put your blog on a short feed. That way they can only ever scrape the feed.