CURD turns sour because of OVER FERMENTATION !!!
Milk has sugar.
Curd bacteria eat this sugar and make acid. This too much acid == sour taste !!!
In warm temperature, bacteria grows superfast.
If the curd is not kept in fridge after 4 - 6 hours, it will go sour overnight !!!
If more than 1 tsp of old curd is added to 1 litre of milk, it ferments too fast and turn sour .