Due to the drought, many of the vegetables gave out early. Now is the time to solarize your soil. Solarizing kills the nematodes that attack many vegetables. If your carrots have double/triple roots, you've got nematodes. If your carrots have little round nodules or balls attached to the roots, you've got nematodes.
The only non-chemical relief is solarizing. Solarizing is placing clear plastic over the area and tamping it down with bricks or boards to keep the heat in. The heat kills the nematodes. Keep it in place at least four weeks and cross your fingers. Hopefully, the soil will heat to a tempurature that will kill subsurface nematodes. No guarantees, but your only organic solution.
If you find that solarizing has not killed enough nematodes to allow you to grow carrots, you may need to resort to raised bed gardens, or you can excavate the infected soil and replace it with new. Both options are lots of work and expense. The final choice is to buy your carrots. I may opt for this one. I've solarized twice, and unless I'm doing it wrong, I still have nematodes.