College Introductory STEM courses impact students' persistence. Guided by Eccles' Expectancy-Value (EV) theory, we examined undergraduates' open-ended explanations for major persistence plans. Students (N=2,737; 45.6% women; 16.4% White, 22.6% Latino/a, 46.5% Asian, 14.5% other; 43% 1st-generation college-going students) reported certainty in their major choice and a subgroup (N = 361) explained why their plans changed at the end of introductory physics and chemistry courses. Nearly half became more or less certain. EV constructs naturally emerged in students' explanation, with decreases in expectancy or values the most frequently mentioned. EV explanations related to whether students became more or less certain about their major choice. Findings underscore the meaningfulness and validity of EV theory for understanding undergraduates' STEM persistence.