It is a little like saying that because IQ scores and missile impacts around a target both obey a Gaussian bell curve, they must share an underlying mechanism. The universality argument is most persuasive when applied to physical systems that have direct analogues to the gradual energy input, dissipative friction, and chain-reaction possibilities of the sandpile. When we come to Bak's efforts to model economic phenomena, however, the universality argument for his SOC hammer practically disappears. He presents no data showing power laws apply to business cycle fluctuations, which are the only phenomenon he attempts to model explicitly. His model has layers of vertically related firms mechanically ordering, delivering, and producing goods.
Firms follow arbitrarily imposed fixed rules for inventory accumulation and exhaustion.
The analogy to falling sand in this model is a series of orders for final goods that arrives each period. The system eventually self-organizes to a critical state where avalanches of production of all sizes are possible, and the avalanches follow a power law. More interesting than the details of the model are Bak's comments about the difficulty of working on it with two economist co-authors, Michael Woodford and Jose Scheinkman. Other social scientists view economists pretty much the way I described the stereotype of theoretical physicists, only without the legitimacy of the atomic bomb to back up their arrogance. Bak seems surprised to find out that economists are actually more careful about mathematical rigor than physicists, and he seems perplexed that his sweeping criticisms of economic theory, pungently expressed but embarrassingly uninformed, were not taken to heart by his economist collaborators.