As no known cause has been given for his death one can only have wonders - however we know his heart stopped and the medical services were not able to resuscitate him.
Unfortunately he is at least the 4th person (male) I have become aware of in mid-late 50's who has suffered sudden death, beforehand seeming perfectly vital and fit. The most recent was Mats Rondin, a Swedish cellist/conductor who died very suddenly in December. I am sure there are scores of deaths like this day across the globe but don't make headlines). Perhaps he was aware of a problem but either was still being investigated or due to busy schedule delaying...the same could have happened listening to Schubert at home.
Every life that gets taken early and unexpected is tragic and sad - to anyone out there aware of heart issues don't hope they will go away. Cardiac care has advanced at mind-breaking speeds and what might have once involved hours of surgery is a day-case.
In addition to his family, the young people in that orchestra are probably feeling the deepest hole in the aftermath and are hopefully getting the support they need.