What would cause a drastic drop in pages crawled per day?

[Fatwallet]

The site didn't go down.

There were no drop in rankings, or traffic.

But we went from averaging 150,000 pages crawled per day, to ~1000 pages crawled per day.

We're now back up to ~100,000 crawled per day, but we went more than a week with only 1000 pages being crawled daily.

The question is, what could cause this drastic (but temporary) reduction in pages crawled?

[Fatwallet]

I wish that were the case, but the site wasn't down.

I looked into the errors, they were redirecting to a subdomain that no longer exists.

[AlanBleiweiss]

So several times in one month the entire site couldn't be reached.  That's pretty significant.  Personally I don't have any clients with that many down-times so can only assume that's the cause or at least a partial cause.  And more important, a red flag that would prompt me to find a better hosting provider if it were my site.

[Fatwallet]

The drop happened March 28th.

There was a "domain name not found" on march 30th (two more on the 22nd, 18th, 12th, and 10th)

[AlanBleiweiss]

There could be several factors.  When did it occur? Did you see any other crawl errors reported?  And unfortunately, the other unknown comes from the fact that Google's own system is both far from perfect and sometimes crawl volume is affected by their own system.

Unless I see crawl errors or an increase in pages not found during or leading up to that period, or more important, see a corresponding significant drop in organic traffic, personally I just chalk it up to the complexity of the web.

[Fatwallet]

Hi Alan!

There were no spikes in kb per day or time spent downloading a page.

[AlanBleiweiss]

Fatwallet

Have you checked Google Webmaster Tools for crawl errors and other metrics?  I had a client recently who had a severe slowdown in their server network which showed up on page crawl speed time as a huge spike - pages loading five times slower than normal.  They subsequently had a dip in pages crawled due to the bottleneck.