It isn't... but the head is made to compress in the birth canal (which is why a newborn has a soft spot in the middle of her skull); sometimes the baby's shoulders are too broad and get stuck.
In some cases, the Obstetrician will even go so far as to try to break the baby's collarbone so that the shoulders will pass through the birth canal. This happened when my younger sister was born in the mid-1980s. She was a 10+ pound baby and arrived too quickly to permit a Caesarian... the OB couldn't break her collarbone, which resulted in the OB pulling at my sister's head and other nurses assisting my mother to push. The doctors almost lost both my mother and my sister -- but our story was a happy one. Both ended up perfectly fine and healthy.
That makes me think that this doctor's maneuver isn't entirely out-of-the-ordinary, though other decisions he made along the way seem to make him liable, and the result was utterly tragic.
http://books.google.com/books/about/Beck_s_obstetrical_practice_and_fetal_me.html?id=3t9sAAAAMAAJ A lady at work bought this book years ago and placed in the library. It’s been taken out thousands of times since by folks with a new one on the way. Everything is here ~ what the doctor did is described in detail.
I have heard of such situations as you describe. I’m glad your mom and sis made it through OK.