Probably due to controls put into place due to company Anti-Money Laundering/Compliance policies. Gift cards are the new way to launder money (it used to be money orders but there are ways to track that now). Criminals use ill gotten funds to buy gift cards (again it used to more commonly be Visa/MC prepaid cards but law enforcement is onto that too) and resell them at a small loss in order to launder the money. My guess is that a PO Box is a red flag in the Disney system as criminals often will open PO Boxes for the purpose of using them for ill intended purposes and then close them up in a short while to avoid detection. This would also apply as a protection in case a criminal is using a hacked card number to purchase GC's and send to a fake address to avoid detection.
You should be able to clear it up by just calling Disney and explaining the situation. Once I had to order something for my DD from Latvia and my credit card kept rejecting it because Latvia is a High Risk Country for money laundering and also because a lot of CC fraud goes through that country. I just had to call my CC company and give permission for that one transaction to go through. I should have know this because I work in the banking Compliance field (however, credit cards are not my specialty).
It really is a shame that this is the world we live in now.
Any clue?
