why does soy milk curdle when salt is added
As noted in "History of Tofu and Tofu Products", alkaline earth metals (such as calcium and magnesium) in solution cause the proteins in soy milk to curdle. As I discovered by searching for "Magnesium in table salt" and "Calcium in table salt", a cup of salt contains about 3 mg of magnesium and 70 mg of calclum. While those concentrations aren't high enough to turn soy milk into full-on tofu, they should be enough to cause some curdling.

All of this hinges, however, on the soy milk being heated. There are a number of soy milk products available in the US (e.g. Silk) that contain added calcium for health purposes--primarily in order to compete with the calcium content of dairy milk--that, themselves, do not curdle because of the low temperatures of storage associated with consumer soy milk.

While some soy starch is contained in the curds of soy milk, the curdling reaction for any solution primarily involves the kinking of the proteins in that solution, trapping within them fats and starches, in order to form a stable matrix of such.