According to media reports the driver was notified of his dismissal the week before Christmas.
The train, comprising four locomotives and 268 cars fully loaded with iron ore, started to roll away from the driver, who had disembarked to inspect an issue on one of the cars, on November 5.
The train then travelled about 92km in about 50 minutes, hitting speeds of up to 110kph before BHP remotely derailed it about 120km south of Port Hedland.
It took BHP about a week to get its iron ore network back to full operation.
On announcing the results of a preliminary investigation BHP Western Australian Iron Ore asset president Edgar Basto said initial findings showed the train came to a stop after a braking system control cable became disconnected.
"The train began to move after the driver had disembarked to carry out an inspection, becoming what is termed a ‘rollaway' train," he said.
Basto said the train was then derailed intentionally because it could not be brought to a stop with the braking system.
"Our initial findings show that the emergency air brake for the entire train was not engaged as required by the relevant operating procedure," he said.
"In addition, the electric braking system was initially stopped the train, automatically released after one hour while the driver was still outside.
Due to the integration failure of the backup braking system, it was not able to deploy successfully."
BHP has put a range of safety controls in place as a result of the early findings.