Arm first- what is baby's position?

A woman's labor is progressing. The water sac releases amniotic fluid. The midwife/doctor examines the woman's cervix. Suddenly, the plan changes from the "routine miracle" of a second birth to an emergency cesarean. Why? The midwife/doctor finds an elbow coming first. Here's the mother's story with slight edits, including those to conceal her identity: 


My 2nd [baby was] 9 lbs 11oz...