After several unsuccessful attempts by an elderly woman to dismount from a horse, the nine-year-old thoroughbred gelding became agitated and suddenly reared up.
It raised its front legs twice and threw the rider off its back, before pinning the 73-year-old under its 450kg weight, a coroner's court heard.
The horse, Goku, had not been fed its afternoon meal and had worked for more than four hours on Nov 14, 2015, when the incident happened at Gallop Stable's Punggol Ranch.
Madam Lim Ah Boey, who remained crushed under the animal for about 10 minutes, died in hospital from multiple injuries.
The court heard that the incident might also have been triggered by event manager Wan Jasma Zuraini Wan Jalani slapping Goku, prior to it being agitated.
She did so after her colleague, Malaysian handler Brian Tan Soon Loong, struggled to help Madam Lim dismount from the horse. He had made four attempts to do so.
The inquiry heard that Madam Lim, her family members and friends had been at the ranch to celebrate her grandson's 21st birthday.
Some time after 5.30pm, Madam Lim mounted Goku while her granddaughter got on another horse.
After a short walk around the riding track, Mr Tan led Goku to the mounting block. But the horse was too far away from the block for Madam Lim to dismount.
This happened three more times; Goku was either not close enough to the mounting block or refused to walk towards it.
Ms Serene Chia, a friend of Madam Lim's granddaughter, saw Mr Tan and Ms Jasma try to pull Goku by its reins towards the mounting block. Ms Chia also saw Ms Jasma strike Goku's rear, but the horse still refused to move towards the mounting block.
Mr Tan and Ms Jasma then decided to walk Goku to an arena about 20m away, where they planned to help Madam Lim dismount using a mobile platform.
But the horse suddenly raised its front legs twice, causing the accident.
In his findings yesterday, State Coroner Marvin Bay said a combination of stressors such as not having been fed its afternoon meal, being hit on the rear and being pulled by the reins would have led to Goku becoming agitated and stressed, which in turn, precipitated its rearing.
Gallop Stable has made changes to the way it operates the rides. It now uses portable mounting platforms instead of the permanent brick mounting ones. It has also implemented a two-person system for joyrides and made compulsory the use of a "neck strap" or halter, which allows handlers on the ground greater ability to control and guide horses.
Said Coroner Bay: "Developing well-thought-through procedures and processes, which are in tandem with those adopted by established international horse and riding societies, would be integral to ensuring the safety of joy-ride customers, many of whom may have little prior experience in riding a horse, and not be attuned to an animal's temperamental shifts, which may predispose to the occurrence of a mishap."
Mr Bay said Madam Lim's demise was a "truly tragic misadventure".