What would cause mental status changes after a cardioversion?

A stroke: There is a risk of a clot getting loose from the heart and if it travels to the brain, a stroke could develop. As medications used for anesthesia are rather short lived it would be unlikely that the medication would cause the mental status change except very transiently.

Depends on timing: Patients will typically still be under effects of sedation after a cardioversion, duration depends on what sedation used. This should improve and resolve, but may take several hours to totally resolve. Also, since cardioversion is done to convert a rhythm problem, most commonly atrial fibrillation, there would be a risk of a small clot leaving the heart and travelling up and causing a stroke.