A.S. Troelstra, Levensbericht A. Heyting, in: Jaarboek, 1980, Amsterdam, pp. 172-175:
Hoewel hij zich na zijn hartaanval wel ontzien moest, genoot hij toch een redelijke gezondheid tot vlak voor zijn dood. Hij overleed tijdens een vakantie te Lugano, Zwitserland, op 9 juli 1980, na een kortstondig ziekbed.
With Google translator:
He died while on vacation ...
One example is Sidney Coleman, though he was a theoretical physicist and not a mathematician. In an interview, he said:
I hate [teaching]. You do it as part of the job. Well, that's of course false ... or maybe more true than false when I say I hate it. ... But I certainly would be just as happy if I had no graduate students. ... Occasionally there is a ...
Sonya Kovalevskaya died at the age of 41, of the 'coronavirus' of her time. From Wikipedia:
In 1889, Kovalevskaya fell in love with Maxim Kovalevsky, a distant relation of her deceased husband, but insisted on not marrying him because she would not be able to settle down and live with him.
Kovalevskaya died of epidemic influenza complicated by pneumonia in ...
These mathematicians (or very mathematical non-mathematicians) died young (<50) and/or from non-natural causes; I did not see these names:
Yom Tov Lipman Lipkin (c.1846-c.1876), smallpox. Codiscoverer of the Peaucellier-Lipkin linkage. A son of the famed Salanter rabbi, founder of the Mussar movement.
Hermann Minkowski (1864-1909), appendicitis. Number ...
No one mentioned John and Alicia Nash (if alicia was a theoretical physicist, then i think she counts), even though they died earlier in 2015, the year the question was posted.
They died in a car accident.