The reason for his Kaieki, punishment, was technically said as the unauthorized marriage of the adopted daughter of Tadachika and Shigenobu YAMAGUCHI, however, there are many theories on the real reasons such as the one by the Incident of Nagayasu OKUBO, or by the conspiracy of Masanobu HONDA and Masazumi HONDA, or by Ieyasu who had the intention to get rid of the Toyotomi government alienated Tadachika who had a close relationship with Saigoku Daimyo (Japanese territorial lord in western Japan) and might present a peace plan (according to the "Tadayo and Tadachika OKUBO, the Lord of the Odawara Domain" written by Kuniteru MITSUGI) but the clear reason has not been known.
改易の理由は表向きには忠隣の養女と山口重信との無断婚姻が理由とされているが、実際の理由は大久保長安事件によるとする説や本多正信・正純による策謀とする説、豊臣政権を一掃しようと考えていた家康が、西国大名と親しく、また和平論を唱える可能性のあった忠隣を遠ざけたとする説（三津木國輝『小田原藩主大久保忠世・忠隣』）など諸説あり、明確な理由は不明である。