Trump’s golfing was not on any official schedule.
Either the perp got lucky or someone leaked the information.
Since the perp has been helping organize and send mercenaries to Ukraine (with CIA permission) I can guess who set this up.
Doesn’t John Brennan still have security clearance?
A couple of assumptions are inherent in the above. First, that this was the only time this guy was there. Second, that Trump doesn't have a routine day and time for playing a round of golf. I understand most golfers have a routine.