In his findings dated May 18 and released yesterday, Mr Crerar noted Ms Cadigan was not speeding and had not been drinking.
It was raining at the time and the car slid across the road into the tree.
Police investigating the crash considered various causes, including aquaplaning, but decided it was most likely Ms Cadigan simply did not see the bend until very late and had to "steer radically", overcorrecting to get back on the right path, he said.
"A reason for this may have been the slice of pizza, which [she] was thought to have been eating at the time, diverting her concentration."
No fault was found with the vehicle.
Two passengers in the car, Ms Cadigan's son Shayne Cadigan-Anderson and her colleague Bevan Eridge, received moderate injuries in the crash, which happened while the trio were returning from Timaru, where they had gone to pick up Mr Cadigan-Anderson.
They were trying to get back to Twizel in time for Ms Cadigan and Mr Eridge to get to work.
Mr Eridge was able to get out of the crashed vehicle, and went to call for help, while Mr Cadigan-Anderson was suspended in the vehicle by his seatbelt and could not get out until the car was rolled back on to its wheels.
Ms Cadigan died at the scene.
In a statement to the coroner, Mr Eridge said Ms Cadigan, who weighed 117kg, did not like to wear a seatbelt because of her big stomach.
The coroner decided that while there had been some question over whether rolling the vehicle back on to its wheels had contributed to Ms Cadigan's death through moving her, it was likely that lying trapped under the weight of the vehicle had done more damage.
"There can therefore not be any criticisms of the passengers and bystanders in affecting the recovery."
A major contributor to her death was that she was not wearing a seatbelt, he said.
If she had been correctly restrained, it was likely she would not have received the type of injuries she did.
"I draw to public attention the need for drivers of vehicles to protect themselves at all times by the use of a seatbelt ... The fact that a seatbelt can be uncomfortable is not an excuse to either break the law or [not] take what is a very simple safety precaution."