Why does milk clot while making yogurt?
The clotting in a yoghurt is indeed caused by the lactic bacteria producing lactic acid and therefore lowering the pH of the milk. However in this particular case (yoghurt making) this is not due to the denaturation of the milk protein, they are in fact intact.
When the pH drop in the milk, it get near the isoelectric point (IEP) of the casein (milk protein) therefore making them partialy neutral (or completly if the pH reach it).
It weaken the electrostatic repulsion between the casein. 
Consequently when the pH get closer to the IEP, casein solubility decrease and often precipitate.  

And this is reversible, if you add caustic soda or another base, the yoghurt or any sour milk will go back to being liquid (well not perfect in some case because the bacteria often produce exopolysaccharide which thicken the milk).