Looking at the http logs, around 5pm an ip address started spamming a whole bunch of requests to the same url. The same ip was doing pretty normal stuff on the website about an hour earlier.

My theory is holding down F5 key would cause the page to reload about 30 times a second. The website was not able to handle that many requests per second and it effectively became a denial of service attack.

This was around 2007-2010 and I think by now browsers have stopped repeating a reload if the F5 key is held down.