It was because NAKAHARA no Chikayoshi detected the movement of the Taira family when he pretended to visit and question them by the order of TAIRA no Tokitada who knew of the relationship between the Hatano clan and Yoritomo.
波多野氏と頼朝の関係を知る平時忠の命令で、親能を召し尋問しようとした平家の動向を察知し逃亡したのである。