Google thought it was a good idea to allow Chromebooks to receive updates from neighboring devices. Instead of each device getting them directly from Google, only one would get them from Google and the rest would get them from neighboring Chromebooks on your network. It does this by sending Multicast requests across your network to request the update locally rather than sending the request out your Internet circuit - hence a multicast storm.
The Pre-fetching is how Google makes their cheap $200 devices appear as fast as a $1,500 MacBook. Each time you visit a web site, the Google browser crawls the entire site and pre-caches any links on the site so when you click on a link, it already has that information pre-loaded and appears fast. If you have a class room with 30 students looking at a site that has links to 10 videos, imagine how much data is being pre-cached, and they may never even click on those videos.
I did a test with this last summer and when 30 devices went to a teacher web page with several links, you could watch the CPU on the AP jump to 80 - 100%. The Chromebooks would then appear to drop off the network. I turned off the Pre-caching and the CPU on the AP stayed at 10 - 15% under the same test.
