The aim of this explanatory study is to identify the causes of intrinsic cognitive load in programming process. For this purpose, a method based on two dimensions; programming knowledge types (syntactic, semantic, and strategic) and programming constructs was proposed. The proposed method was tested with high school students enrolled in Computer Science course dealing with Python programming language. The results suggested that the difficulties determined by the proposed method were considerably useful to estimate the cognitive load experienced by the students. The results also confirmed that when the errors and the difficulties encountered in the programming process increased, the element interactivity in programming tasks increased in terms of syntactic, semantic and strategic knowledge as well. We hope that this study will contribute to the efforts for decreasing the cognitive load to facilitate programming learning process.