This was because he was informed by then that Mitsunari had virtually supported Hideyori and Osaka-jo Castle (at the stage of the Oyama meeting, he was informed that Mitsunari started his forces independently), and he was afraid that the daimyo might rush to support Mitsunari.
これは、三成が大坂城と秀頼を事実上擁立していることが伝わったため（小山評定の段階では三成の単独挙兵として伝わっていた）、彼らが三成のもとへ駆けつけることを恐れたためである。