I used the google.com/ping service to make it download 500 sitemaps for a customer of mine, that was about 30 hours ago.
Googlebot reacted immediately, downloaded all 500 sitemaps.
Now I have more than 30,000 downloads from Googlebot in less than two days.
Always the same unchanged sitemaps are downloaded, again and again and again. They are valid, clean and Bing downloads them ONE time and is fine with it.
I am quite worried about that amount of downloads, that will cause significant traffic over time and it's useless.
Those sitemaps are not going to change soon.
The webserver is a normal Apache.
The download count increased to 92000 downloads of unchanged sitemaps, I blocked google from access to sitemaps for half a day. This helped reduce the rate but it is slowly ramping up again.
Though since I blocked the GoogleBot the rates are a lot better than before.
So at this point it seems under control but the incident is really strange.