An excerpt from "Guitar" magazine, 11th January, 1993 in which Ron Eve did an article about being Mark's tech:
"Mark is certainly a hard taskmaster and has high expectations and is perhaps a little intolerant to things going wrong."
"For example once he walked on, put his foot on the Ernie Ball volume pedal and the string mechanism broke. Now I'd checked that before the show - I couldn't predict it - and he got a bit upset!"
Reading between the lines, maybe there was a bit of friction, or tension between them?
Perhaps, because DS had come to an end, he just decided to retire!