Response-shift bias explains how changes in students' criteria for self-evaluation can suppress the effect of educational interventions in pre-to-post shifts of self-reported metrics, such as the Colorado Learning Attitudes about Science Survey (CLASS). Therefore, the static or declining pre-to-post shifts typically seen in the CLASS might not reflect development in students' reasoning about themselves based on their acquired knowledge of physics. We observe response-shift bias in a downshift between pre-instruction responses obtained at the beginning of the semester and retrospective pre-instruction responses obtained at the end of the semester. We investigated possible causes of response-shift bias in the CLASS by adding a free response question after select CLASS items during a two-pass survey administration, asking students to explain their reasoning behind their expected and perceived shifts. We present the themes that occurred most clearly in students' free responses to one survey item, which highlight the established forms of response-shift bias: recalibration of the students' standards of measurement, reprioritization of the students' values, and reconceptualization of physics content.