This happens from time to time as things come into the US through customs. It is a complete pain in the ass and is your government at work, a part of the beloved Department of Homeland Security. You will probably have to pay an import duty to have your stuff released. As far as I can tell, it is random.
"The Internet has made it easy to find and purchase items from almost anywhere in the world. However, many people are discovering that getting a foreign-bought item successfully delivered to the United States is much more complicated.
When goods move from any foreign country to the United States, they are being IMPORTED. There are specific rules and regulations that govern the act of importing - and they can be extremely complex and confusing - and costly."