Also happens when a hot new product or technology gets announced and the first couple of people that see the announcement begin vying to be first in line. The night that I heard that Chase's BlinkCard was announced, I was beat to the punch on the .com, but had the .net in my cart. I took 2 minutes to check the status of 3-4 other names, and when I went to check out, the net had been regged and was no longer avail. Over the next few hours, there was a chase on for Blink technology names that was intense but amusing at the same time. ("Blink" refers to RFID technology as it applies to a "blinker" that the customer passes a "touchless" credit card in front of, in lieu of of the card being "swiped" by the checkout person at the sales counter.)
Although I wasn't involved in the Vista "chase" on day one, I imagine that a similar scenario took place on day/hour one, after the announcement.