What you describe sounds very confusing and must be very hard for you to deal with. It does not make sense that he would change so drastically under the circumstances you describe. Like your post alludes to, you are searching for a triggering event to make sense of it. A full medical workup would rule out anything physiological causing the change and barring that therapy to look for psychological triggers that would explain the change. Him being unwilling to seach for a cause leaves you powerless to do anything.