I'm not sure if this has been addressed before, but if you are approached by a faction to reduce CA prior to being coronated and are then coronated after acquiescing, crown authority will reset to its previous level from the last ruler.
For example, my previous ruler had set CA to Absolute. He died pretty young and my heir took the throne but could not be crowned as I had insufficient funds. This locked CA to Low. My court faction approached me to lower CA, and I acquiesced as my army retinue had just been dwindled in a civil war and I didn't have the resources to fight them. CA was reduced from Low to Minimum, but upon becoming coronated it reset to Absolute Crown Authority rendering the faction's previous ultimatum worthless.