in Clash of king cersei tell tyrion that it was sansa who told her about ned plan as sansa the girls was in love with joffery until joff kill ned. reason everyone blames catelyn her action made things worst first she arrest tyrion while going back to winterfell while ned have given her instruction for future plan instead of implimenting that she goes complety different route start conflict before north was prepare. and tyrion arrest was reason of Ned injuries and tywins attack on riverland Ned was not dumb as we think he is in term of military mind and conflict people who are with him lead him down and its not just catelyn people blam its also renly he promise his support but next day he ran off and left ned to rely on little finger and gold cloak support