No reason is mentioned in Nihonshoki (Chronicles of Japan), but it is possible that the Prince Otsu's followers hid the Prince Otsu's identify because he was a minor, and a follower falsely called himself as Yamabe no Okimi since they could not identify friend or foe.
理由は書紀に明記されないが、敵味方不明の地を行く際に、少年だった大津皇子の存在を隠し、従う者の誰かが山部王らの名を騙った可能性がある。