Usually its because the phone # or address you provided doesn't match the bank records exactly. I had that happen when I tried to order a refurb Epson 6500UB, which have been extremely limited in availability from the Epson Store. My order was cancelled without any warning and of course, it was out of stock. I called and it turned out I gave them my home phone, but my bank has my cell phone listed as primary #. Yep, they had a hold on my card , but it cleared after 3 days. Luckily for me, I found another one available on the website the next day and snatched it, made sure the phone number was correct and it went through.
That is exactly the reason why I don't make major internet purchases using my debit card. I've had too many 'holds', etc.