Cabbage loves a lot of water and it is better to water it at the root, and not on top of the heads of cabbage. Perhaps there was a drought, so the top leaves were all blackened and dried out. But it's not so scary, the main thing is to remove all the top leaves and see what's inside?
And also the reason may be in insect pests that spoil the roots of cabbage, all kinds of larvae, a bear. Gotta clean the ground