I had a full term stillbirth apparently caused by a very small placental abruption. What may have caused this? What are the chances of reoccurrence?

Reocurrence. I am so sorry about the loss of your baby. Some studies have shown recurrence rates to be one in eight, while some have reported a ten fold increase in rate. Whether or not this recurs depends upon the causation and on the presence of the predisposing factors: pregnancy induced hypertension or chronic hypertension, external trauma, cigarette smoking and/or the use of Cocaine and alcohol.
How terribly sad! As you know, abruption occurs when the placenta separates from the uterine wall too early - instead of after birth. Some drugs may be related to placental abruption, e.g., cocaine. There are also some blood clotting problems which may be related to placental malfunction. Talk with your doctor now about measures to minimize risk with your next delivery, i.e., planned c-section when lungs are mature.