Well, from my knowledge and after referring to my "Bible" this generally means TOO MUCH nutrient. Or too much of one thing in your nutrients. As your plants obviously can't talk to you i'd suggest doing what KoKomama had stated and do a flush for about a week and you should see drastic improvement in leaves looking not sooo curly.. Whenever I start to see it in my babies I give a day or two of just water w/ Molasses (my secret ingredient) the sugar should help break up any salt buildup around the roots too..