Recently experienced a 50% drop in traffic and one of the only things I changed is the sitemaps.xml
We have 9 sitemaps with 50k URL each, I added the following formatting
<URL>http://url.com</URL> <lastmod>2016-04-29T01:55:23-04:00</lastmod> <priority>1.0</priority>
Before it only contained the URL.
I read here that adding a timestamp may have negative impact, but the author was talking about the post itself, not the sitemap. http://www.shoutmeloud.com/effect-of-showing-date-stamp-on-blog-traffic-case-study.html
I know it's counterintuitive, like why would it have a negative impact? But that's the only thing that was recently changed.
Also the amount of indexed pages jumped from 500k to 2 million during the same period, so may be the 2 are related? Some kind of a temporary Google dance?