chapotraphouse

That was part of it too, but (leaving aside Brian entirely), Shaddam was a man whose will was severly restrained by the Great Convention, particularly Landsraad was organisation which main or one of main purpose was even explicitly stated in the book by Thufir Hawat iirc - their greatest fear was that Emperor would use the Sardaukar to kill them off one by one, and it was stated to be the reason why the masquerade with Harkonnen uniforms was done. And it was HUGE risk for the emperor, he wouldn't dare to do that for no reason, and certainly not when he had Fenring as advisor.

So there was much more of it between them. Also Leto himself was pretty keen to pursue this course so he wasn't just a victim, by all probabilities he really was aiming at Shaddam. Possible most obvious course was 4 daughters and 0 sons of Shaddam, popular and politically savvy duke could easily push for marrying his son and heir to Irulan and became the all powerful force behind the throne. Shaddam would certainly fear that scenario, but i bet it was still not all. Well we will never know now, Brian version of the events are not even coherent with Brian version.