The 747 was taxiing to the active RWY, and due to the taxiway having quite a bit of downslope the crew was using idle reverse in order to avoid riding the brakes. Problem is, the 747 retracts the LE flaps when reverse thrust is applied. Apparently this can be overridden somehow (sorry for the lack of details, but I´m not exactly an expert about the systems of a 747, I´m sure someone else will correct me if I´m wrong), but for some reason the crew neglected to do so.
The leading edge devices take 7 seconds to extend with T/O flaps selected so they would have been well and truly out long before VR. The East African Community Accident report stated the
cause of the accident was the failure to select the pneumatic power source for LE device extension.