In recent years, the metaphor of technical debt has received considerable attention, especially from the agile community. Still, despite the fact that agile practices are increasingly used in critical domains, to the best of our knowledge, there are no studies investigating the occurrence of technical debt in critical software development projects. The results of an exploratory field study conducted across several projects reveal that a variety of business and environmental factors cause the occurrence of technical debt in critical domains. Using Grounded Theory method, these factors are categorized as ambiguity of requirement, diversity of projects, inadequate knowledge management, and resource constraints to form a theoretical model. Following previous studies we suggest that integrating agile practices, such as iterative development, review meetings, and continuous testing, into common plandriven processes enables development teams to better identify and manage technical debt.