This sort of thing apparently happens because Google thinks you're sending out lots of searches from your IP address. Bot attacks can resemble this sort of thing too. It also happens a lot when there's heavy Internet usage, e.g. at the time of Michael Jackson's death.
In the last week or so I believe there have been a number of Google spiders roaming the Net and latching onto some sites, including the RavesceneBBS
and another BBS which I administrate (it happened there first possibly because I was linking to it in my signature a lot and I was posting here a lot, hence multiple instances of that link) causing a sudden jump in the number of users online at any one time (our BBSes are very small in popularity, so this is noticeable).