It's important to start by saying chocolate manufacturers don't add Butyric Acid. In the States, some manufacturers (but not all) use the enzyme lipase in the production process.
This enzyme starts the process of lipolysis and during this process, Butyric Acid is produced. This acid prevents the milk, and therefore the chocolate, from souring. Other foods BA is found in include hard cheeses like Parmesan.
It is also present in butter, milk, full fat yogurt and cream and in fermented foods like Sauerkraut and miso due to bacterial fermentation.
Butyric Acid is also found vomit and obviously has unpleasant associations.