Its regular practice for Google to make updates to their algorithm throughout the year, but they sure do keep things secretive.
Only a day before did we see an announcement made on Twitter publicly notifying us of the update.
The info I'll summarise below is based on research compiled by Ezoic, which consisted of monitoring 5000 publisher websites over a 10 day period.
What was the June Google Core update?
Generally Google updates make changes to how Google ranks sites on searches and indexes these sites. In Googles twitter announcement Google mentioned they were releasing a "Diversity Update". The idea behind this is to reduce the same site appearing twice after your search.
Here' s what Google said on Twitter:
"Have you ever done a search and gotten many listings all from the same site in the top results? We've heard your feedback about this and wanting more variety. A new change now launching in Google Search is designed to provide more site diversity in our results…. "
Why did sites lose traffic
There were a number of commonalities that cropped up in this research where sites lost traffic. Here's a summary of what to look out for:
A low mobile page load speed. Google can help you check your page speed with their free tool. https://developers.google.com/speed/pagespeed/insights/
Out of date content and stock images. A Google Webmaster team member had previously mentioned this.
Which sites were negatively effected
So some background here.... back in March, Google had an update which was named the "Medic Update " because of how it effected Health and Wellness sites. Have a look and see how Google is interested in sites that effect Your Money or Your Life (YMYL), here's along read about this in Googles Search Quality Rater Guidelines . To get this right there are a couple things you need to bed down on your site specifically expertise, trustworthiness and authority, maybe one day I can write a bit more on this subject.
Some interesting notes that may help your site:
Keep your content fresh and regularly updated
Non English sites are effected more with the update
Sites with affiliate links on every page were negatively effected.