Maybe they were hairline cracks that you didn't see before putting it in the incubator and the motion of turning caused it to open more?
I have cracked an egg by accident when I put it back into the incubator too firmly before.
I do have a little wax baby in my incubator that so far is doing great! The crack was there when I got the egg and I decided to seal it with wax and incubate it anyway to see what happens. It's officially on lockdown and should hatch on Thursday! Fingers crossed yours pulls through as well!