Old, unpasteurized orange juice has extra tang
It is almost certainly a fermentation - opened juice does not keep for a week in the fridge. So you got some bacteria in it which are creating tangy byproducts (lactic or acetic acid). It is a method of creating fermented drinks such as cider (or even fruit wine), but without following a tested process, you cannot be sure if some of the bacteria may be harmful. So it is not safe by today's standards.