Oranges could be responsible. Not being negative here, because I do believe in the OJ fast. But a lot of opinions in the alternative community tend to be skewed, so nobody's going to blame the fruits.
Just to bring a little balance to the discussion, it could be impurities, or it could be enamel erosion. Many people who do juice fasts find the fruit acids harsh on their teeth, and over time the enamel erodes.
It would be hard to tell without a proper checking. For all you know maybe a ton of tarter/plaque came out! But in the future, if you do juice fasts, rinse your mouth with water/baking soda to neutralize acids.