Teaching mathematics through mathematical modelling involves interacting with students’ representational systems and ways of thinking. Because student reasoning is often non-normative, incomplete, and self-contradictory from the perspective of the observer, the question How should an observer respond to students’ work? arises. The prominent approaches to resolving this question tend to use descriptive framings of students’ modelling work and observers’ knowledge, which fall short of informing contingent responses. Instead, I argue we must attend to Why did the student model the way they did? and use explanatory accounts to link observers’ moves to students’ work. In this chapter, I present and reflect on vignettes of students’ reasoning during mathematical modelling to articulate my position and urge the field to shift away from descriptive accounts of students’ modelling competencies towards explanatory and prescriptive framings of their reasoning.