This large-scale study aimed to investigate the impacts of underlying factors on lecturers' burnout in emergency online classrooms during the outbreak of the COVID-19 pandemic. The study attracted the participation of 399 lecturers conducting online emergency classrooms from 30 universities and colleges across Vietnam. Data analyses with EFA, CFA, and SEM indicated that such factors as support resources, anxiety towards emergency online teaching and Coronavirus, lecturer's technological and pedagogical content, and knowledge significantly impacted their burnout levels. In contrast, no significant difference in burnout states was found between lecturers with different demographic features, genders, and residences. The results from this study also suggested critical pedagogical implications for higher education leaders and administrators to prepare emergency online classes for sustained education in times of crisis.