Google suddenly indexing 1,000 fewer pages. Why?

[hoosteeno]

We have a site, blog.example.org, and another site, www.example.org.

The most visited pages on www.example.org were redesigned; the redesign landed May 8. I would expect this change to have some effect on organic rank and conversions. But what I see is surprising; I can't believe it's related, but I mention this just in case.

Between April 30 and May 7, Google stopped indexing roughly 1,000 pages on www.example.org, and roughly 3,000 pages on blog.example.org. In both cases the number of pages that fell out of the index represents appx. 15% of the overall number of pages.

What would cause Google to suddenly stop indexing thousands of pages on two different subdomains? I'm just looking for ideas to dig into; no suggestion would be too basic. FWIW, the site is localized into dozens of languages.

[willcritchlow]

Off the top of my head, some things to check:

• noindex tags deployed from staging during the redesign
• broken rel=canonical / hreflang
• update to robots.txt
• broken status codes (are those pages still 200 OK status?)
• site speed - has the performance of the site as a whole taken a hit? It needn't necessarily be about those pages specifically, but could be about the site health as a whole
• changes to XML sitemaps

Some other harder to check / track down causes:

• changes to internal link architecture
• removal of pages that were the target of powerful inbound links

Hope something there helps - good luck!