Bicyclists are not generally known for shooting each other. Apparently there were witnesses, since there is a close description of the shooting and the appearance and clothing of the shooter. This may have been an ambush by someone who knew the doc's commuting habits, but a pro would presumably have picked some more secluded location. So would an intelligent amateur. The circumstances suggest a spur of the moment shooting. Maybe the doc and the shooter had just had a riding incident and a random Mr. Unstable snapped.
Of course, the description says the shooter was a white or Hispanic male. Could be drug or gang related. There are 1001 ways for doctors to get sideways with druggies. The shooter was wearing a warmup jacket in 100 degree heat. For all we know, he was covering up his MS-13 tats. The point being: we don’t have a clue.