To help diagnose the cause in a year to year decrease in organic search, we need to look at more than just the overall traffic numbers.

First, take a look at the landing pages. Is there one particular landing page that is losing most of the traffic, or is the loss site-wide? Is there a blog post that had been getting traffic for some reason that has now stopped? This can happen when a page shows up in an answer box in Google and then stops.

Use this information in conjunction with Search Console. Look at Google Trends for keywords for those pages to see if demand has decreased.