Google is pushing some “core updates” which they describe as “significant, broad changes to our search algorithms and systems”. Their advice to users should their site drop in rankings?
Focus on content
…pages that drop after a core update don’t have anything wrong to fix. This said, we understand those who do less well after a core update change may still feel they need to do something. We suggest focusing on ensuring you’re offering the best content you can. That’s what our algorithms seek to reward.
Be sure to check out the full post for additional advice.