Instagram decided it needed to shake things up in order to compete with other image sharing networks. The image sharing app has decided it is going to restructure the way that users see their feeds stating that "...people miss on average 70% of their feeds... This means you often don't see the posts you might care about the most." The idea is to keep users on your app as much as possible, and Instagram believes it can do it by mixing up your feed every now and again. This could be most effective when users don't follow too many people and end up seeing the same 5 posts in the past hour...
Source: http://www.gsmarena.com/instagram_is_going_the_way_of_twitter_algorithmbased_feed_to_replace_chronology-news-17248.php
