The reason for the procedure was that accessory gearbox was not covered by the fire extinguishers and low px would sometimes end in a fire. They did provide it with a fire detection system though!
Another reason was if the AGB lost oil pressure and seized, the drive shaft would continue the rotate. It would invariably begin to flail around and in the process beat the snot out of the two upper burner cans, at least that's how it was explained to us. Every F-27 I flew - admittedly not a lot - all except one also had gearbox oil pressure gauges. I forget at what pressure the warning lights came on and I'm too lazy to go look it up. You could also get a fire warning on the F-27 after shutdown if the airplane was parked down wind. Wind blowing up the tailpipe could cause enough hot airflow over the AGB fire detection system to activate the warning.
Pailot77 I think I would say the actions of the crew were all good. They got the airplane back on the ground in spite of major structural failure. That to me says they acted in a very highly professional manner given the situation in which they found themselves. Two things, did the engine autofeather during the loss of the prop and has the engine been torn down yet? On teardown verify the HP and IP turbine blades for signs of liquid aluminum spatter.