No reason mentioned in "Nihonshoki" (Chronicles of Japan), but it is possible that the Prince Otsu's followers hid the Prince Otsu's identity, and someone pretended to be Yamabe no Okimi, since entering the areas 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。