Marisela Echeverria, 36, was dragged beneath the bus and ultimately run over by its rear wheels, a Los Angeles County Sheriffs official said.
The bus was heading eastbound on PCH at the time of the crash.
The bus driver told authorities the woman appeared to lose control of her bike as vehicles passed by.
Investigators believe she lost control and struck her handlebars on a parked vehicle, causing her to veer into the side of the bus, according to sheriff's Sgt. Philip Brooks.
She was then dragged under the bus and run over by the rear tires, Brooks said.
"She was on a ride, training for Ironman Arizona," said Brad Hall, Echeverria's coach and friend.
He had been out training with her earlier in the day during the first half of her 80-mile ride.
This was the second time Echeverria was gearing up for an Ironman. She missed the first one with the rest of her team seven weeks ago becasue of her father's death.
"She had initially trained with Team in Training for Ironman Canada, and tragically her father passed away the night before, so she was unable to complete that race," Hall said.
According to Hall, despite the grief she was dealing with, Echeverria was determined to fight on and complete her first Ironman by the end of this year.
"We chose Ironman Arizona to be the race that she honored herself and her father," Hall explained. "She trains to raise money for The Leukemia & Lymphoma Society."
Hall says that, on Saturday, several cars were parked too close to the road, not leaving enough space for bikes to get by safely.
He encourages people to keep this in mind the next time they're parking along PCH.
The driver of the bus was not arrested. All MTA buses have video recording on board, and investigators will be reviewing the tape, police said.