Could be a variety of reasons.
I'm no specialist, but from managing my own site, I've been advised that sometimes web-searching 'bots' can visit a page many times in one go. Or, it could be someone refreshing the page, or perhaps something on TV or Radio encouraged the surge in visitors.
Perhaps there really were that number of people visiting? Does it say the time of day that people visited, and is it the same IP address?
As I said, I'm no specialist, but I'm sure there's a reason for it.
It's most likely to be because this forum really is the best in the world, and the visitor numbers are only a sample of what's to come in future!