Clearly had possibly previously unidentifed heart disease issues.
A good mate of mine dropped dead on a tennis court a few years ago , fit as a fiddle for mid 50’s, ran 5kms almost every day, ate well, drank little ( very little) , ran a business but not overly stressful, happy marriage, on no medication and had 6 months medical check ups, and really everything to live for with his kids grown up and he and his wife just about to start travelling.
They found out during the tests after his death that he had a heart defect that had never showed up prior.
A sad day for the family and way too young to die!