This study, which aimed to investigate qualitatively the factors that contribute to the difficulties encountered by non-Physics majors teaching Physics, serve as the basis for competency enhancement for teaching STEM. The study was limited to thirty (30) non-Physics major teachers teaching Physics from nine (9) public and private schools in the coastal areas of Iligan City. Grounded Theory Methodology and Thematic Analysis were used in analyzing the data. The study found out that there are five (5) factors that contribute to the difficulties encountered by non-Physics majors in teaching Physics. These are: Facilities and Equipment; Sufficiency of Learning Materials and References; Teachers Readiness and Attitude towards teaching Physics; Learners’ Readiness and Attitude towards Physics; and Curriculum. Based on the findings, Reasons of Difficulty Theory was formulated. Non-Physics majors had difficulties in teaching Physics due to some factors which are embodied in the three (3) elements of this theory. These are Readiness, Resources, and Curriculum.