It's been thirty years since I last flew the Canberra but I seem to remember that there was an inbuilt delay in the ejection sequence so that an explosive link could allow the control column to go fully forward to clear the pilots' knees. Possibly this short hiatus was the difference between the nav's successful escape and the two guys up front.
A sad day, as always, but calling the Canberra dangerous is overstating the case. It has a vulnerability to power loss on takeoff due to an appreciable difference between (in airline terms) Vr and V2 and loss of speed on final approach was potentially fatal. These were known problems and every Canberra man trained assiduously in these areas and thought about them on every flight. Viewed against sortie rate it was no more or less dangerous than any other military combat jet. I'd fly one again tomorrow.