I may have an answer to this mystery. In the book The Call of the Hen, written by Walter Hogan and published in 1921, the Author talks about crooked pelvic bones. Here is some of what he has to say:
Presently we hear the whole flock take up the chorus, and going to see what the trouble is, we find the
hens holding an "Old Maids' Convention" and declaring they will never lay another egg, it hurts them so much to do so. On examining them, we find the pelvic bones so crooked they come together like the horns on a Jersey cow, and when the hens lay, instead of the vent dropping down between the pelvic bones, allowing the egg to be released in an easy manner in a few minutes after the hen goes on the nest, the egg is forced to be delivered between the pelvic bones and tail bone, thus prolonging the agony of the hen sometimes for hours, when,if she was built right, as in Fig. 34, she would be relieved of the egg without pain in a few minutes. And instead of wasting vitality in getting relieved of the egg, she would be rustling
around for material to build another one, and thus add at least 20 per cent to her egg-producing value.
This matter of crooked pelvic bones is more frequent in some breeds than in others,and is a serious matter that is very easily remedied by breeding only from birds with the straightest pelvic bones; especially looking after the male birds, as one male bird with crooked pelvic bones will transmit this defect to all of his daughters.
When I came to Petaluma, I found whole flocks of thousands of hens with crooked bones; now they are very rare. The poultry-breeders soon caught on to my straight-and-thin-pelvic bone idea; and I think the Society for the Prevention of Cruelty to Animals should recognize my services in relieving millions of hens of the agony of parturition.
After reading this book, I would recommend this book to anyone interested in breeding chickens. It is a good read.
For anyone interested in reading this book you can get it from the California Digital Library for free as a PDF file at this link:
http://archive.org/details/callhenscience00hogarichBill