The following factors contribute to ups and downs or website ranking:
No 1 is common and inevitable and affect millions of website worldwide. When any alogorithmn is changed it will no longer provide ranking credits to the given keyword criteria and hence many pages will be dropped off from the top ranking through many keywords.
No 2 is normally when the website contents is changed or altered by owner or webmaster . By changing the web page content it means you are instructing search engine to re rank the page basing on the new ranking criteria and in many cases it will end up score less points enough to be at the top.
No 3 When the server become slow or unavailable due to attack, failure or prolonged service/software upgrading it will prohibit search engines bots from indexing your pages and thus ranking may drop
No. 4 When the website is under attack or hacked it won’t perform well because many webpages may be unavailable or load very slowly or even deleted by hackers in the extent that search engine bots cannot index the contents and hence will be dropped from top ranking.
No 5 Introducing or changing the scripts in the system by updating or altering may cause website malfunction.
New script may conflict with the server or the core scripts in the website or even with other scripts. In other cases the script generates html tags that make the page load slow or violate the html tags standards. Other security scripts generates code in the robots file or htaccess and hence your web pages may be affected either by error 404, 403 or even 500
no 6 it depend on you or your SEO company agreement
Any ranking website need a continuous online marketing. this include constant exposure of your web pages with products or services to the social network to increase clicks, submit to directories, forums, blogs etc. Marketing via video and articles. When this stops, the website ranking will also stop because many valuable ranking points are gained through specific web page clicks. more clicks more points.
no 6 it depend on your competitor trick and techniques
Just like no. 6 above, if you competitor is serious and is performing a heavy online marketing, you should also do the same and even above them. If that is not implemented, then your competitors will be on top of you in terms of top ranking.