Same thing happened to me, no idea why.
So, I just had Best Buy and my bank on the phone at the same time and my bank placed the hold on the card because it was a purchase for over 200 coming from an electronic store - specifically - best buy is flagged as a retailer to watch for fraud from.
Any way, hold lifted, pre-order reconfirmed and shipping should remain on schedule. Still not expecting to see the switch until next week, but that is fine.
The nice lady on the BestBuy supported said that she's never seen anything like this in terms of customer inquiries on the phones.
These are going to be hard to get for a few weeks.