Having spent 6yrs on Hawk and same on Tornado during a 24 yr career as an Armourer I am very familiar with the Mk10 seat. The statement from the Armourer SNCO from the Reds is correct. The AP used to contain a statement to the effect of 'tighten the stiff nut until 1 1/2 threads are visible through the nut' or words to that effect. This was checked on both the Vital and Independent checks.
I have always felt that the failure to release in this accident must have been caused by this stiff nut being overtightened, as this would prevent the Scissor Shackle from opening correctly.
As for why did the seat fire in the first place? The Seat Pan Firing Unit sits very close to the cockpit floor when the Seat Pan is motored down on its Actuator. A reasonable size loose article (for example a Mobile Phone) could be enough to dislodge the Sear in the Firing Unit and initiate the ejection sequence.
As rightly pointed out by other posters, serious accidents are often caused by several events lining up to cause a disaster (Swiss Cheese Theory). If they were operating at reduced manpower and/or experience levels, that could explain why an otherwise very reliable system has been catastrophically compromised, leading to tragic results.
I am convinced some people in the service will know the cause of this tragedy, I hope it will come out.