Anaphora, as an important linguistic phenomenon, represents a cohesive relationship concerning two parts, namely antecedent and anaphor. The choice of anaphoric forms is understudied in previous research. In this study, an annotation framework is built and a machine learning method is employed to analyse the influence of motivators on the choice of anaphoric forms. In addition, the framework of accessibility theory is modified, with causals as the study object. Results indicate that competition-, salience- and distance-related motivators, as well as text type, can significantly influence the variation of anaphoric forms. Among those motivators, predictability emerges as the most significant variable. Under the influence of these motivators, zero pronouns, noun phrases and pronouns exhibit significant differences in distribution. Pronouns have a broader distribution range and fewer restrictions compared to zero pronouns and noun phrases. Based on the results, we also modify the accessibility theory in terms of competition and salience.