Investigating the factors that drive requirements change is an important prerequisite for understanding the nature of requirements volatility. This increased understanding will improve the process of requirements change management. This paper mainly focuses on change analysis to identify and characterize the causes of requirements volatility. We apply a causal analysis method on change request data to develop a taxonomy of change. This taxonomy allows us to identify and trace the problems, reasons and sources of changes. Adopting an industrial case study approach, our findings reveal that the main causes of requirements volatility were changes in customer needs (or market demands), developers' increased understanding of the products, and changes in the organization policy. During the development process, we also examined the extent of requirements volatility and discovered that the rate of volatility was high at the time of requirements specification completion and while functional specification reviews were conducted.