To help their children obtain higher scores in English and prepare them for a better future, many Vietnamese parents are likely to send them to a commercial English language center (CELC) for private lessons. Accordingly, the quality of CELC instructors’ teaching receives remarkable attention. It is crucial to explore what factors affect their teaching instructions as well as their choice of instructional strategies. This qualitative study used Grounded Theory to gauge a deep understanding of factors affecting 21 CELC teachers’ choices of teaching techniques which directly decide whether their teaching is good or bad. The data from 21 semi-structured interviews revealed that factors, including teaching conditions, teachers’ sense of technique’s effectiveness, teaching experience, time allotted, formal and informal professional training and development, and stakeholders’ influence, significantly affected their choice of teaching techniques used in the classroom. Additionally, the Grounded Theory study indicated that teaching beliefs were more influential, even above other factors. These beliefs can be tentatively classified into (i) conviction of the effectiveness of the teaching technique, (ii) pressure from stakeholders to use a particular teaching technique, and (iii) contextual factors that may affect the selection of techniques. The article discusses the findings in light of the theory of planned behavior and Vietnamese educational and socio-cultural values. It has implications for teacher training and teacher professional development.