Why does ranch salad dressing made with buttermilk separate into water on bottom?

Ranch salad dressing made with buttermilk separates because of the difference in densities between the various ingredients. When stored, the heavier solids and emulsifiers in the dressing settle, causing the liquid components, including water, to separate and rise to the top. Additionally, without sufficient emulsification, the oil and water components can break apart over time, leading to separation. Shaking or stirring the dressing before use can help re-emulsify it.