In order to understand probability, you have to take things as coming from a random event and people don't clearly have that in their mind from an intuitive standpoint.
Such decisions are therefore rational, as opposed to a reaction to a randomevent, because traders believe the existence of momentum is a reason for the stock price to rise higher.
According to an ongoing investigation by the Washington Post, who initiated the McColo shutdown, the Srizbi bots have been programmed with a mathematical formula that in the event of a shutdown generate a random but unique web address to check for updates.