This was because the conditions of the pacification between Tokugawa and Hojo included an article that Tokugawa should rule Shinano and Hojo should rule Kozuke, and it is also said to be because Masayuki--who had territories in Numata City, in Kozuke, in addition to Shinano--was afraid of losing his own territory there.
これは、先に徳川・北条間で結ばれた和議の条件に、信濃は徳川が、上野は北条がそれぞれ支配するという内容が盛り込まれていたことによるが、信濃の他に上野の沼田市にも領地を持つ昌幸が、自領が失われることを恐れたためといわれている。