There are several causes for misshapen or hairy roots. Overwatering, hard soil, and overcrowding all contribute to forked or missahpen roots. Root knot nematodes can also cause misshapen roots.
Plant carrots in loose, evenly moist, not soggy, soil. Thin plants as they grow to avoid overcrowding.
Management of root knot nematodes is difficult. Planting resistant varieties and practicing crop rotation can help.
Read more about management of nematodes.