I admit without reservation that the following is nothing more than considered speculation based on what I have read on these pages and what I have observed in similar cases in the simulator.
Aircraft has compressor stall left engine very close to VR. Witnesses report flames seen from left engine which are a characteristic of compressor stall. Reason for compressor stall unknown yet. Indications of compressor stall in cockpit may include hearing bang and slight yaw and some vibration. Vr is called and pilot (captain or first officer?) instinctively starts rotation then changes his mind and de-rotates. Someone pulls hard reverse on both engines as part of abort procedure. Probably gets reverse on right engine but possibly not on left engine depending if actual engine failure. Aircraft pulls right under influence of full reverse right engine and fatal delay in countering the right departure from runway centreline. Depending who was PF at the time, critical delay in handing over control to captain who presumably would have initiated abort process. Lack of good braking on grass or hard dirt verges extends stopping distance already compromised by abort action during rotation. Pure speculation of course but fits the reported track of aircraft.