Very simply put: you trained your body to use its own stores instead of expecting food during your fasting time.
When you stopped IF, your insulin & counterregulatory hormone responses re-aligned, and your body no longer anticipates that need to pull glucose from internal stores and release into the bloodstream in order to function.
(This is all of course assuming you have no medical conditions, take any medications or supplements, etc that could potentially negate or modify the bodily processes I mentioned above).