The fact that he was sent to hospice, a place for people nearing death, and not a nursing home, where dementia patients are sent, indicates they weren't telling you the whole truth. Dementia patients can live for years. My mother, for instance, 20 years.
Doctors tend to avoid discussions of death, and many, at best, allude to what is occurring: end of life. Lots of buzz on honest end of life notification these days, because patients and their families often would make different decisions if they'd been informed (bucket list).
Many brain impact diseases can at best be a guess, along with causes, which may or may not be diagnosed via autopsy. You're likely to get clearer answers on "what exactly happened" by researching the decline cerebral atherosclerosis patients. Such things are usually a list of cascading physical failures, and not just one.