He was killed by an automobile driver. The U.S. is notorious [1] for bicyclists (and pedestrians) being hit by cars at a relatively high rate, because it prioritizes automobile infrastructure and speed (and culture) over bicycle and pedestrian infrastructure/accessibility/safety.
[1] Not to say it's the worst, just relatively bad for an advanced country.
[1] Not to say it's the worst, just relatively bad for an advanced country.