The Windows Update servers have been under pretty serious stress over the last couple of days. Ironically, by encouraging everyone to turn on Automatic Updates, Microsoft have built a distributed Denial-of-Service system - that attacks their own site.
The Automatic Updates tool checks for any updates whenever your network is idle. (Anyone whose connection is charged per-byte should now go and turn it off!) If it finds any, it starts downloading them in the background, depending on how it's configured.
Those of us east of the US were probably out of the office when the latest set of updates were posted, at 10am PST, or (IIRC) 5pm GMT. Everyone whose computer was turned on queried the servers, found the updates, and started downloading...
If you're in a networked environment, you should look into getting Windows Update Services/Software Update Services. For administrators, this allows staging and testing of updates before deploying to workstations. It should reduce your network bandwidth as well as alleviating the load on the servers.