heat, ph, a total shlew of reasons. but looking at your grow here it could also be the intensity of the HID light you have on them. using HID lighting to grow newly sprouted seedlings is like forcing a todler to run.. dont worry though looks like they will be fine. i dont know what your watering cycle is but it could also be over watering. like a i said a total shlew of different things it could be.. but after i looked at your journal i would say dont worry about it!!