General question: A calendar printing company notices a significant decrease in sales over the past years. Which marketing research design should be used by Marketing researchers to identify "why"? Descriptive or casual? Which marketing research methods should be used?

No further information required - this is just to determine which marketing research design is to use in order to identify a significant decrease in sales.

I propose to do exploratory research first (should be part of almost every research project) and I'm unsure which marketing research design to use after, descriptive or casual.