That's what the Civil Aviation Authority wants to find out.
It's launched a probe into the incident after the aircraft carrying 32 passengers left the airstrip and came to a halt about 40-metres.
None of the passengers or crew members was hurt - although some were left traumatised and shaken.
Passengers on the East London-bound flight - have told medics that they heard a loud noise from the left of the plane - as it rumbled down the runway and gathered speed.
It then spun violently and skidded onto the grass.
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