"Such a common retaining wall with such little design reference. No wonder they fail so commonly."
It is suggested that they fail NOT because little is known or published about how to design them but rather because:
Wood is a natural material with widely varied properties
Not all important properties can be predicted
These properties change with time and loading
wood can be attacked by insects, water, weather, plants (tree roots for example), chemicals, rot, and burrowing animals
good quality, durable heart grade wood is expensive and hard to find
walls are often subject to overloads, such as heavy equipment, which were not anticipated in the design