If you're going to have a glass of orange juice and brush your teeth (a good idea), there's only one order in which to do them. It makes sense that your minty toothpaste-mouth would make OJ taste weird, but why so down-right heinous?
As Bytesize Science explains, it's because your tongue gets coated with a particularly troublesome foamy oral detergent. It's about as tasty as it sounds. But whatever the reason, it's not the worst thing in the world; you should be brushing your teeth after breakfast anyway, you freak. [Bytesize Science]