He was on his fourth run of the day when he lost control of the bike.
"After three runs you do get tired but he seemed happy with his times and was a mature competitor," Mr Thompson said.
It was the first fatality in the history of the annual event, although some riders had received serious injuries in crashes in past years.
"The road was closed and we stopped racing immediately after the crash and our condolences go out to the family of the rider," Mr Thompson said.
He said lone racers contested the time trial events and "can go as fast or slow as they please - they're in charge of the throttle and there's no one pushing from behind".