Google has said this before, but it is worth repeating, that an increase in crawl rates may be a sign that you are switching over to mobile-first indexing. John Mueller repeated it again on Twitter the other day and below are some stats to show you what you might see.
John responded to a complaint about a huge increase in crawl rates saying "Looks like a shift to mobile indexing. We try to reprocess things on mobile as quickly as possible. If it's too much, use the search Console settings."
Looks like a shift to mobile indexing :-)
— 🍌 John 🍌 (@JohnMu) October 9, 2018
We try to reprocess things on mobile as quickly as possible. If it's too much, use the search Console settings.
It might not be 100% the case but I looked up some of the crawl rates for sites that got notices on September 20th and I saw most of them, not all of them, saw huge spikes in their crawl rates on September 8th and/or 9th.
Here are some of those crawl rate charts showing the spikes for those sites with notifications on September 20th:
Compare that to a site that was not switched over yet and you see this:
Forum discussion at Twitter.
