I did meet a former examiner at an IOSH event last month. He said the reason he stopped was because many of the examiners are academics who have little real world experience and are pedantic in the extreme.
The marking process is that each Marker gets around 10 papers to mark. After marking they all meet up and discuss the marking to verify the consistency across the Markers.
He jacked it in when at one such verification meeting they were discussing the marking of a question where 1 point was available for listing the equipment to be worn for forestry work.
He was told he couldn't give a candidate the mark because the person had included in their answer, the term 'safety boots' rather than 'forestry boots'. All the other equipment was listed correctly, but they were arguing over the term 'safety boots'. So he said 'stuff it' and stopped being a Marker.