Yes, fasting can cause an increase in blood ketone readings. Fasting has shown to increase insulin sensitivity on a cellular level and secrete glucagon on a systemic level. There is some evidence that this combination of hormonal responses to fasting gives rise to ketone levels in the body. There is also some evidence to support that fasting can lead your cells to use ketones as fuel faster than if you weren’t fasting.
This rise in blood ketones in response to fasting is an old survival mechanism. When pre-historic humans were hunter/gatherers, this mechanism helped them to survive days at a time without eating, as well as helped to keep them sharp in-case of any danger, or in the case that they had to outsmart and hunt an animal. It was a survival mechanism.
This response occurs because your body is attempting to store as much glycogen and muscle as it possibly can in response to a fast, so it will tap into fatty acid stores, and use the ketones as a fuel source, and as a protective shield to preserve as much protein as possible. A rise in ketones can also help your brain to function better by being more alert, and more focused.