Information Technology projects have migrated toward two dominant Project Management (PM) methodologies. Plan-driven practices provide organizational control through highly structured plans, schedules, and specifications that facilitate oversight by hierarchical bureaucracies. In contrast, agile practices emphasize empowered teams using flexible methods aligned to organizational values reinforced by cultural controls and rituals. While project teams mix and combine practices from both traditions, a stylistic bias remains in the initial project management practices established at the onset of IT projects. A review of existing literature on IT project management serve as a filter on the stylistic biases of these dominant PM paradigms that is organized as a two dimensional PM Style framework. This study employs a two-phase multi-method approach that builds on this framework to develop and test a theoretical explanation for the influence of inevitable unexpected challenges that emerge to disrupt IT projects. The first phase is a comparative case study contrasting two agile projects with two plan-driven projects. A Critical Realist method organizes the events and structures revealed in the case study with insights from existing theories of Loosely Coupled Systems to propose a new theory of Reactive Decoupling that describes the shift in PM style triggered by unexpected challenges. A confirmatory study using survey techniques collecting data from 268 professional IT project managers is analyzed to provide empirical support for the new theory. Findings support the theory's proposition that fluctuations in the form of unexpected challenges unfreeze the baseline project management style and encourage a shift toward decoupled systems behavior. This shift allows some plan-driven projects to behave as loosely coupled systems and gain the performance advantages of greater requisite variety and flexibility. Where the intensity of fluctuations is sufficiently great, the shift may drive projects out of the loosely coupled systems arena toward fully decoupled systems status, where results are dependent upon the self-control and good-will of ad hoc project team behaviors. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]