Microservices migrations are challenging and expensive projects with many decisions that need to be made in a multitude of dimensions. Existing research tends to focus on technical issues and decisions (e.g., how to split services). Equally important organizational or business issues and their relations with technical aspects often remain out of scope or on a high level of abstraction. The objective of this study is to holistically chart the decision-making that happens on all dimensions of a migration project towards microservices. We investigate 16 migration cases, by conducting a grounded theory interview study with 19 participants that recently underwent a migration. We also provide an initial validation via a Web-based survey with 52 respondents. Our study approaches the topic with a strong focus on the human aspect of a migration, through stakeholders, their concerns and the decisions they need to make as part of the migration. We identify 3 decision-making processes consisting of 22 decision-points in total, and their typical alternatives or options. The decision-points are related to creating stakeholder engagement and assessing feasibility, technical implementation, and organizational restructuring. Our study provides an initial theory of decision-making in migrations to microservices, and outfits practitioners with a roadmap of which decisions they should be prepared to make and at which point in the migration.