Without having any real idea of the origins of the phrase (which I agree is remarkably apposite in most of the situations where I've come across it) and disregarding anyone who says, "What's new?" I have always understood it to mean that events had become so erratic that there shape was no longer perfect. Perfect is often - usually? - epitomised by the circle: "The perfect circle," which, in turn, can be equated to apple shapes. Pear shaped, therefore, could mean that the apple-like perfection has been lost.

Mind you, personally, I tend to the view that all the best things come in pears.