Donations to charity in general often cause credit cards to be blocked -- my wife and I do this thing[1] where we give large amounts to multiple charities on the same day each year, and last year it took a couple days to get all the charges to be successful and cards unblocked.

I think the idea is that credit card thieves use charitable donations to figure out whether they have a valid card, so they're considered especially suspicious[2].

[1]: http://blog.printf.net/articles/2012/11/27/celebrating-seven...

[2]: http://www.networkworld.com/news/2007/070607-credit-card-thi...