Hi Cee Dubb, and welcome from one stranger to another. Day 1 and posting #3 - I'm on a roll after weeks of being only a voyeur.
I'm going out on a limb by not looking it up, and worse, I'm risking being politically incorrect, but....

I've always assumed its origin is to do with body shape - particularly of women. The perfect body shape, the marketers would have us believe, is the hour-glass. If your body is trending towards having more weight around the hips and bottom, you're "going pear-shaped", i.e. going in an undesirable direction, going to pieces, going to seed.

I hasten to add that the sentiments expressed by the coiner (?) of the expression are not necessarily those of this writer. [Insert some anxious hours here, awaiting retribution/rebuttal/excommunication/worse.]