None of the pledges get charged until after the campaign ends - that is one of Kickstarter's marketing points. In this case, the large pledge doesn't go through when Kickstarter tries to charge it. Incorrect payment info given, expired card, not a high enough limit, or something similar happens and the backer doesn't fix it in the week they are given after the campaign ends to do so. For the really large failures, Kickstarter has waived their own rule and has given the backer even more time to find a solution, but it still hasn't worked. My impression is that the people who never respond to the communications from Kickstarter or who pledged the large amount and knew it wouldn't go through are the ones who get banned.
There are methods to deal with that, where the payment has already been validated and authorized but it has not yet gone through. I should know, as I've implemented them at my job. Basically, the transaction gets validated with something like paypal and the site gets a key, that can be used to actually execute the transaction later on. Depending on bank and payment service, it could be that the funds are not frozen in advance so the bank account may not have enough money when you execute the payment, but normally this kind of things should not happen.