"Overly sour yogurt is a sign of inconsistent inoculation" - What is inoculation?
In this case, inoculation refers to the introduction of yoghurt bacteria (lactic acid bacteria) that transform the milk into yoghurt.

If anything goes wrong - from unclean conditions that let other bacteria grow to unfavourable temperatures that inhibit the proper growth of your desired lactobacillae - you speak of improper inoculation.

The effects on taste and possibly food-safety have been discussed in your linked posts.

So work in a clean environment, use a good strain of bacteria and maintain the recommended temperature range to prevent this.