Lack of SOP, poor crew cockpit discipline, complacency (being at home airfield) fatigue....
These are all issues which could of allowed the crew to overlook their position on the approach. (if that is what happened - prime candidate IMO)
Factor in that some of the NAVAIDs were not working... don't hear a QNH correctly etc etc and you soon end up in a situation where you could do something silly.
Good SOP's are aimed to catch things from slipping through the net - but they depend on cockpit discipline.
PT6A