At certain times the traffic to the website gets declined after reaching a high. Even without much change in the website files the traffic gets declined over a period of time. Most of the data can be seen through various Analytics platforms giving statistical data about the decline in visits. Most commonly the following might be the top reasons for decline in traffic for a website.
Competition
One of the main reason for a website showing lowering of keyword ranking is because of the entry of competitor websites in ranking. Checking about the statistics and characteristics of own website alone will not be enough in such cases. Periodically checking about competitor websites and showing up of their results in the Search Engine Results Page should be seen as an indicator in the decline of visits.
Content
The content on the website need to be evergreen to attract the customers at any time. When the content gets outdated regular readers will stop reading and something new has to be published in order to retain the audience. One way to find whether the same content that is published earlier is drawing audience regularly. Republishing old content and giving updated version is a way to stop such declines.
Search Engine Algorithm
Sometimes Search Engines change algorithms and this lowers the rank of existing web pages. Web Spam reporting by competitors along with such algorithm changes can drastically reduce the views for a particular web page. It is advisable to constantly watch the search engine algorithm updates and make the content structure and other parameters of the web page compliant with latest standards.