One theory is that due to the great efforts by SAKANOUE no Tamuramaro, major opposing powers within Emishi were removed, which produced many subordinate groups, the rapid increase of which was a threat to order in certain places.
坂上田村麻呂らの活躍によって蝦夷の主だった抵抗勢力が排除されたために大量の服属民が生み出されたためにその急激な増加によって現地における支配・統治に不安がもたれたからとする説。