Murphy's Law has no place anywhere near ejection seats. It's been a few years since I was working on Tornado F3s as an Airframe Technician, but the reverence with which the seats and associated systems are treated by the groundcrew should not be underestimated.
I would seriously doubt that the tightening of a scissor shackle bolt would be the last stage of seat servicing in a bay. Whilst I would not wish to pre-empt any coroner's findings, I would be shocked if it turned out that the scissor shackle bolt had been tightened to the extent that it was the only thing preventing the parachute deploying.
The cause of the ejection sequence itself will hopefully be revealed, and lessons learned.