The aircraft was almost new, well maintained, and a docile, predictable trainer. The instructor was very experienced, conservative, an admirable promotor of safety culture and (from personal knowledge) pretty much guaranteed to have set trim (FUST) before half way downwind. Also hard to believe that airspeed would be permitted to decay on final. Bear in mind that when landing a glider, one must virtually fly it almost into the ground. There is always a good margin of safety with airspeed.
I can only surmise that an extraordinary event occurred late on final.
Nothing more can be guessed at.
Anything more IS just guesswork, and risks unfairly maligning our lost comrade.
Please be respectful with posts.