Why does curd become stringy or creamy at times?
You are dealing with the proteins in milk and sometimes when they bind up they form chains, hence the strings. Are you stirring them a lot? That would do it, as would having certain substances in the the mixture that would affect how it separates.
In yoghurt production, why does the mixture become acidic?
The bacteria used to make yogurt (Lactobacillus) produce the lactic acid as a by-product of anaerobic respiration (pyruvate is converted to lactic acid through lactic acid fermentation). They are able to survive in the lower pH environment, whereas other species of bacteria have trouble dealing with the acidity. This gives the Lactobacillus an edge in competition with other species in the yogurt. When pH reaches a critically low point, it serves as an indicator to the bacteria in the yogurt that available sugars are running low and that the levels are inadequate to handle further growth, so the colonies halt their cell division until conditions improve.