Why SEO Changes Happen?
This is a big question, why do SEO algorithm changes happen, why can’t Google be static?
Well, the answer is, for searcher’s convenience. That’s why Google rolls out an update just to refresh its search index and to look for websites involved in unethical SEO practices that violate her quality guidelines. If the search engine finds a website violating Google’s quality guidelines, it may penalize it by ranking it down in its SERP, or sometimes completely removing it from Google’s search index.
Google’s update check for the things like
Black Hat SEO usage
Extensive link building
Google launched several Panda and Penguin Updates