The Benchmark is an event happening every 5 days. One possible is that your group was off and many during the weekend passed the 5 day period, it thus probably they would all do that near simultaneous.

But, don't know exactly how this Deep Freeze works. If BOINC perceives it as being a fresh install or upgrade, the benchmarks would also be run. There are a few other members that run this environment, so maybe they can comment.

cheers

Added: Here's thread discussing Deep Freeze and the re-registration, which would explain the benchmarking: http://www.worldcommunitygrid.org/forums/wcg/viewthread?thread=11349