I managed to get some more information from my siblings and hospital staff. Dad was drinking very heavily starting at noon or even earlier. I had no idea how bad it got because I haven't seen him in 4 years and was never invited or allowed to visit. Apparently, he was drinking like this daily since mom died 3 years ago which explains why I was frozen out.
Alcohol is indeed cunning and baffling. Dad was frustrated with my mom's drinking for years. Mom would pressure him to drink with her and my dad often refused and would leave the room to work on his hobby. He had never had a drink before he met my mom as medical students. With my struggles with alcohol and my mom drinking herself to death, I am very surprised my dad became an alcoholic this late in life.
Hope everyone is doing well! Thanks for letting me vent!