User blog comment:CoolJazzman/Soken Ishida death caused by Yhwach?/@comment-8295743-20130626010130

It was directly stated that Mayuri caused his death, he purposely delayed the Shinigami from arriving to slay the Hollows that attacked Soken. Soken was a very old man(He looks to be in his mid 70s at least), he probably couldn't handle a whole group at once anymore. Hell, the group of Hollows attacking Soken was probably Mayuri's doing as well.

I do not think the Shinigami knew of Yhwach's awakening before hand. Otherwise, why not launch a first strike? There was a period of time between Soken's death and Aizen's betrayal, and then 17 months(Maybe 18? Anyone know over how many days the Fullbring thing took place?) between Aizen's capture and the initial attack by the Vandenreich.