I think the incident you refer to was a RAF Harrier on air test from BAE Dunsfold with a test pilot ( T Scott (RN) RIP) and it was never proved what happened as the seat was never recovered. Although post the accident blocks were put around the saet linkages to stop them from being accidently bent and firing the seat uncommanded. One scenario was that the seat might have been being lowered over an unknown obstruction that bent the seat linkages (preventing the correct firing sequence being followed).
I was there at the time.