Background: As engineering becomes increasingly incorporated into precollege classrooms, it is important to explore students' ability to engage in engineering practices. One of these practices, "engaging in argument from evidence," has been well studied in science education. However, it has not yet been fully explored in engineering education. Purpose: This study aims to identify the classroom situations that prompt students to justify their engineering design ideas and decisions. The following research question guided the study: "What initiates the need for fifth- to eighth-grade students to use evidence-based reasoning (EBR) while they are generating solutions to engineering design problems in engineering design-based science, technology, engineering, and mathematics (STEM) integration units?" Methods: Within the naturalistic inquiry methodology, we analyzed student team audio recordings from the implementation of seven different engineering design-based STEM integration curricula across three school districts to identify instances of EBR and categorize the situations that led to them. Results: This analysis produced seven categories of situations that prompted students to use EBR. Two of these categories, "responding to adult" and "documenting," were teacher-prompted; students frequently justified their design ideas and decisions when talking with adults or responding to prompts on worksheets. The other five categories were student-directed: "negotiating," "correcting," "validating," "clarifying with team," and "sharing." These categories occurred without direct prompts from adults or documents. Conclusions: This study offers implications for teachers and curriculum developers about how to explicitly integrate scaffolds for EBR into design-based curricula as well as what situations teachers can look for to observe student-directed use of EBR.