You're growing medium must be very rich. I know this sounds paradoxical, but carrots get gnarly like this when over fertilized.

Carrots need a very fine tilth and grow best in a sandy soil. The will, naturally, bifurcate if the root finds the smallest stone! I suspect this is what has happened to your carrots.

Also, were they grown from seed or transplants? Lately, some garden centers have been offering transplants, but if the plants have been sitting in those little cells even a few days too long, that is what you get.