Why did my potatoes turn orange after I shredded them for hash browns?

I was shredding three amish potatoes for a batch of hash browns and they immediately started turning an orange color when exposed to the air. Any clue as to what is causing this unusual oxidation effect?

Are you sure they aren't Amish Bush Porto Rico sweet potatoes? They have a pink exterior and an orange interior.

If that's not the case, read this:

http://www.open2.net/everwonderedfood/potatosecrets.html