Donut-style Glaze Cracking
I’m assuming you’re using a standard recipe and process (where you dip the dougnut in the warm glaze).

If your cooling environment is quite cold, rapid cooling of the glaze can cause cracking. Another popular reason is movement, once placed on the cooling rack, it is advised that you don’t move them around.

A third reason I can think of is inadequate liquid. If all above fails you should try slightly increasing your liquid ingredients.