I used about half of a quart of Silk Milk and returned it to my refrigerator. Two days later the remaining liquid had been replaced with a colloidal mass with a gooey consistency not unlike rubber cement. Why? I see no signs that anything froze in the refrigerator which was in continual use.
Soy milk can spoil, just like ordinary cow's milk. That seems to be what happened.
While spoilage in cow's milk is usually souring, and the smell is unmistakable at fifty paces, soy milk spoils by turning gooey. I'm not sure about the details, but it's something in how the proteins react to oxygen. In soy milk, if I'm not mistaken, they turn into longer, stickier chains.
As for why it spoiled, it was probably just open for too long. Even in the fridge, soy milk has a life of about 4-6 days once opened. The seal may have been damaged somehow even before you actually opened it, allowing air into the carton to cause the spoilage.
Tofu is manufactured by coagulating proteins in soymilk with magnesium sulfate. As bonding occurs between the positively charged magnesium ions and negatively charged anionic groups of the protein molecules, the proteins coagulate.
Since magnesium sulfate, also known as Epsom salt, is a common household item, anyone with a cursory knowledge on the matter can turn soy milk into the base for tofu with little effort. Perhaps a prankish roommate?
Do you have soy yogurt in the same fridge? Soymilk based yogurt with live cultures, if somehow able to cross contaminate your milk, could cause fermentation to occur. I've never used soy milk, but I know the same process for making regular yogurt is the same as the process for making soy yogurt, just with a different set of cultures. Short answer - it sounds like some buggies may have gotten into it. Give it a taste and see if it's pleasant or not.