I'm not sure if low
nutrient concentrations will cause the tomato's to ripen specifically. But low nutrient levels is certainly stress for the plant, and stress in general will trigger the plant into reproduction. Basically it's just trying to produce
seeds for offspring before it dies. Heat is stress as well, and it may not even be one or the other. It can be a combination of things, including all the different stress the plant has been through in the past.