An HSE investigation found Baker had initially attached a winch to the front of the Range Rover, which would have prevented the vehicle rolling away if the handbrake was released or failed.
But Baker later removed the winch, jacked up the vehicle and placed skates beneath the rear wheels, it said.
Because the wheels were not secured with chocks, and no breakaway cables were attached, there was nothing preventing the vehicle from rolling backwards when it was lowered on to the skates.
The investigation concluded Baker failed to take reasonable care for his own safety and that of others by not following established recovery procedures.
The HSE also found his actions fell below the standard expected of a competent and experienced recovery driver.
Baker later pleaded guilty to breaching Section 7 of the Health and Safety at Work etc. Act 1974.
Speaking after the sentencing hearing, HSE inspector Alexander Ashen said: “Sean Baker was an experienced recovery driver.
“Failing to properly secure a vehicle weighing more than two tonnes on a steep hill was a fundamental error.
“This happened in a busy town centre, close to restaurants and car parks, with vehicles, motorcyclists and pedestrians regularly passing through the area.
“Sean Baker’s failings were substantial, they exposed everyone nearby to risk and resulted in a member of the public, who stepped in to help, suffering life-changing injuries.”
Additional reporting by PA