Causal reasoning is a fundamental cognitive process that supports a wide range of learning activities such as making predictions and inferences, explaining phenomena, and drawing conclusions. In physics learning, causal reasoning plays an important role in developing conceptual understanding and problem-solving skills. This study builds off the existing work to investigate aspects of students' causal reasoning in understanding Newton's third law. Quantitative assessment is conducted to explicitly probe four attributes of causal reasoning including the causal relation between a pair of interaction forces, magnitudes of interaction forces, the time order of interaction forces, and the action-reaction language. Interviews are also conducted to identify details of students' reasoning pathways. The subjects of this study include high school and college students from China. The results suggest that after instruction on Newton's third law many students still hold a belief of causal relation between the pair of interaction forces. Further analysis on students' response patterns and interviews suggests that students' belief in causality is strongly associated with asymmetries in scenario contexts due to dominant features, the scenario timeline, and the action-reaction language. In addition, the results also demonstrate a progression of four developmental levels of causal reasoning that each reveals a set of unique thinking pathways for using the asymmetries in reasoning to determine causality. The findings of this study suggest that the lack of correct causal reasoning may cause students to develop fragmented conceptual understanding, which should be addressed in future research and instruction. Detailed analysis of the findings and educational implications on assessment and instruction are discussed.