My condolences go out to the family and friends of the deceased, and also to the rest of the Sqn. But, sadly, tragic accidents like this can and do happen.
Anyone who has flown an ac sitting on any ejection seat, or the armourers/safety equipment personnel who work on them, know how to treat them with respect. Whilst being a potential life-safer they can also be lethal and a seat should only be armed pre take off and then made safe again after landing.
Inadvertant ejections have happened before, both on the line or in the hangar. A zero, zero seat should have worked in this senario (stationary ac, level attitude) but only if the pilot is properly strapped-in and connected with leg restraints, PEC etc. This was the case of a Canadian student pilot who ejected from a Harvard II a couple of years ago. He lived.
It was sadly not the case when a Harrier pilot died in similar circumstances following a display in the 1980's which was well documented at the time and written up in Air Clues. Whatever the cause in this case it is a complete tragedy but only the BoI (sorry SI now) will be able to determine what actually went wrong.
MB