Quote below from another forum which sounds like a sensible theory... Could severe rotor or some similar weather phenomenon have contributed to an airframe failure? I would have thought they might have had time to bail out. Very sad.
They were last seen at 14K over Mount Rose few minutes before they were observed 5 miles east at around 9500 feet when the glider broke apart either due to flutter or high G. From Mt Rose they were suppose to go west to start the task as it was a local contest day. So we know they had no reason to dive 5 miles east and do acro. Neither of them was known to do acro, especially not low in the mountains and during a contest. What was observed by the witnesses is a glider out of control at its final stage before breaking up in the air which can certainly look like aerobatics. Similarly they had no reason to try an airstart high over the mountains. The flutter noise may have sounded like a motor.
That's all we know and I doubt we will ever know more as the fire consumed the glider.