There are a whole load of factors that can result in a site slipping in Google’s listing. Here are some potential reasons:
1. Deficiencies or changes in your website or SEO – quite simply this can be based on all sorts of factors such as duplicate pages, internal link structure, internal link anchor text, external links, external link anchor text, page load time, number of pages, amount of text on pages, use of images/video, social shares, number of incoming links, quality of incoming links, appropriate use of title and description tags, correct use of h1, h2 and h3 tags, use of image alt tags, age of site, freshness of site. Changes to your site may be positive for SEO, but could also prove negative. Also changes to the sites that link to you can have a positive or negative effect that are completely outside your control. Extreme deficiencies in your site may result in a penalty from Google.
2. Other websites improving their site or SEO. There may be nothing wrong with your site or SEO – it’s just that other sites have improved and so their ranking improves (all of 1 above but as it applies to other sites)
3. A change in Google algorithm – Google are constantly changing their algo but sometimes they introduce a bigger more significant change that has a notable impact. Sometimes if the algo change doesn’t seem to have a beneficial result in Google’s view then they may roll back or partially roll back the changes. Sometimes one SEO factor may be more important but then it becomes less important.
4. From time to time Google seems to like to shake up the results a bit to see if people get a better user experience if they rank different sites more highly.
5. Changes to a site and new SEO – sometimes positive changes seem to have a short term negative effect (days or maybe weeks) before a more mid term improvement.
Find out more about Your true Google Position
Find out why you shouldn’t put all your eggs in the Google basket