You didn't consider any filter element against case ground, so the common mode voltage carried by the supply lines will be "copied" to the PCB ground, even across an "isolating" DC/DC converter. The PCB ground becomes hot against the shield, or viewn from the PCB side, the board is surrounded by a hot shield which injects interferences to any critical node through the case-to-board capacitance. In return, through the same coupling mechanism. PCB bound EMI will be transmitted to the outside, conducted as common mode interfernce by the supply cable.