Given the parsimony of information provided by SMRT - they even declined to confirm the nationality of the bus driver - one can think of several possibilities:
He stepped on the accelerator instead of the brakes;
The tyres were bald, and wheels lost traction;
Bus was travelling too fast to make the 90-degree turn;
Brake line was cut due to sabotage by disgruntled employee (skid marks on the road make this a highly dubious scenario);
F-1 is hiring.
Sheer speculation of course, fuelled by lack of disclosure and trust in public institutions.
Quick on the defensive, Kalai Natarajan, Vice President of Corporate and Marketing Communications at SMRT covered all the standard bases: "SMRT has a fleet of 1,137 buses, which undergo strict and thorough preventative maintenance checks. These include mandatory LTA inspection repair, as well as predictive and corrective maintenance of components such as braking and steering mechanisms. The bus involved in this morning's accident was last maintained on June 21, and was given a clean bill of health. No brake or steering related problems were noted during the inspection."
One passenger, trapped in the bus, was pronounced dead at the scene by paramedics, and his body had to be extracted from the mangled bus.
Another death of an individual by a negligent act not amounting to culpable homicide. Not to worry, there's a new precedent, fine is $10,000.
Don't waste money on an independent Committee of Inquiry to look into the circumstances surrounding the accident, and to recommend improvements to existing systems, protocols and processes. The outcome is a foregone conclusion. The "buck stops here" sign is definitely not on the CEO's desk. Zhang Kun may insist his brakes failed, but the system will beg to differ.