Why are lithium batteries stored at 50% voltage and not a lower voltage?
I've always assumed that the lower the voltage, the less material
  degradation, but that starting a storage cycle with a voltage too low
  runs the risk of dropping the voltage to a point where the cell cannot
  be recharged, but I haven't been able to find a definitive reference
  for this.


A definitive answer is difficult because few studies have been done that are freely available, and the results from one particular cell type may not be applicable to others. 

I found a study on Aging of Lithium-Ion Batteries in Electric Vehicles which tested Panasonic NCR18650PD Li-ion cells. The results show that storing at 3.45 V causes less degradation than 3.7 V, so your assumption is correct for normal operating voltages. At 3.45 V the remaining capacity is very small (probably less than 1%), so your other assumption (that leaving too little charge may risk dropping into over-discharge) is also correct.

      

Another interesting thing they found was a big jump in degradation above 55% charge. Storing at 50% charge was not much worse than lower voltages, but 60% and above was much worse. This was attributed to changes in chemical composition of the cell at different charge states, so a cell with different chemistry may not react the same.