I think the pitot tube in the aircraft failed.
It was giving incorrect air speed for the pilot to take off, it must have been under reporting the air speed, making the trainee to increase the speed of taxing and thereby coming to the point of take off but not able to do so. The ejected pilot must have been the HM subject, where as the pilot still trying to cope with a/c must be the young IAF chap.
( Its analogous to me ready to bail out while I was teaching my daughter to drive during her high school years, as to the twin ejection system, my car too had two independent air bags that could deploy at time, hope they would have luckily I did see that eventuality).
all this is conjecture, only court of inquiry can reveal, as to the rusted part I am asuming the Pitot-Static (Prandtl) Tube (prandtl assembly has one part heated to keep ice clogging hence could have corroded).
Anyways read on here
According to an investigation by Argentine and Uruguayan Air Forces, the pitot tube - the primary instrument for measuring the aircraft's airspeed - froze when the aicraft passed through a cloud, blocking the instrument and causing it to give a false reading. Compounding this problem was the failure of the alarm designed to report such a malfunction (raising serious questions about inspection irregularities by the Argentine Air Force). Thinking that the aircraft was flying at dangerously low speeds, the pilots increased power to the engines. Far from flying at the low speed reported by the instruments however, the aircraft was actually flying well outside its safe cruising speed, and far above a safe speed for deploying slats. During the deployment of the slats, one was ripped off by the force of the high speed airflow travelling over the wing, which caused the plane to become unflyable and enter a steep descent.
During the descent, the black box recorded an increase in the plane's speed from 300 to 800 km/h in three seconds, which could only signify the sudden unfreezing of the pitot tube. Specialists predicted that the plane crashed perpendicularly to the ground at a speed of 1200 km/h, leaving a crater 70 metres wide and 10 deep
Airspeed is a measurement of the plane's speed relative to the air around it. The pitot (pronounced pee-toe) static tube system is an ingenious device used by airplanes and boats for measuring forward speed. The device is really a differential pressure gauge and was invented by Henri Pitot in 1732. An example of an air pressure gauge is a tire pressure gauge.
The open end of the pitot tube, usually mounted on a wing, faces toward the flow of air or water. The air speed indicator actually measures the difference between a static sensor not in the air stream and a sensor (pitot tube) in the air stream. When the airplane is standing still, the pressure in each tube is equal and the air speed indicator shows zero. The rush of air in flight causes a pressure differential between the static tube and the pitot tube. The pressure differential makes the pointer on the air speed indicator move. An increase in forward speed raises the pressure at the end of the pitot tube. In turn, the air pressure pushes against a flexible diaphragm that moves a connected mechanical pointer on the face of the indicator. The indicator is calibrated to compensate for winds in the air or the speed of the opposing current in the water. In airplanes, electronics also compensate for altitude and air temperature to make the air speed measurement accurate.
I could be totally wrong