I don't know this factually for this specific instance, but often times, the big moves seen in "hostnames hosted" are caused by domain name registrars changing platforms. For exaple, GoDaddy serves an ad billboard page for every expiered or un-configured domain registered with them. This results in a huge number of these pages, so when/if someone like GoDaddy moves from, say, IIS to Apache (just an example), you'll see a big swing in the charts.

These kinds of statistics are interesting to look at, but they don't mean much to me. I select a web server based on suitability to my needs, not "what are all the cool kids using".