Frank,
I know nothing about the equipment that you are working on but I do know SCRs. Perhaps the SCR had been triggered before the board was restarted. The SCR would not turn off until its cathode current was near zero. You will also find that higher current SCRs will not remain on after removal of gate drive, below a minimal cathode current.
If the circuit checks current through or voltage across the SCR on boot up, it might signal a shorted SCR.
Joe