Continuous integration, delivery and deployment (CI/CD) is used to support the collaborative software development process. CI/CD tools automate a wide range of activities in the development workflow such as testing, linting, updating dependencies, creating and deploying releases, and so on. Previous quantitative studies have revealed important changes in the landscape of CI/CD usage, with the increasing popularity of cloud-based services, and many software projects migrating to other CI/CD tools. In order to understand the reasons behind these changes in CI/CD usage, this paper presents a qualitative study based on in-depth interviews with 22 experienced software practitioners reporting on their usage, co-usage and migration of 31 different CI/CD tools. Following an inductive and deductive coding process, we analyse the interviews and found a high amount of competition between CI/CD tools. We observe multiple reasons for co-using different CI/CD tools within the same project, and we identify the main reasons and detractors for migrating to different alternatives. Among all reported migrations, we observe a clear trend of migrations away from Travis and migrations towards GitHub Actions and we identify the main reasons behind them.