Why did my coat of spray paint crack along a profile line?
Might be caused by the second layer shrinking beneath the third layer because it hadn't fully dried.
In high humidity conditions, I generally increase the recommended drying time between recoats by 25%, just to be safe.

Alternatively, if the backside of the door has been stripped and hasn't been resealed, the door itself may be expanding slightly due to the humidity.

I haven't actually had this happen before on something like a cabinet door, so these are just guesses based on experience with painting outdoors.