"In order to defend itself against the cold, [carrots have] developed all these amazing physiological responses, including increasing the sugar content. Increasing the sugar content helps defend against ice crystal formation, which can do all kinds of terrible things to cells like dehydrate them, crush them, rupture them. ... this increase in sugar content helps defend the carrot against frost and cold." UCLA's Liz Roth-JohnsonThis is called "cold sweetening" and happens to lots of vegetables. I knew it happened with parsnips, kale, and Brussels sprouts. I just didn't expect these carrots to be so sweet!
The variety is Mokum. I planted in the middle of August, worrying that it was too late in the season to get a crop. I'll make this date for the future and will plant lots more of them!