First two possibilities to come to mind:
a) Kate is MKULTRA victim and one "alter" was programmed to harm POTUS but other alters were telling her not to. In the heat of the moment, a psychological meltdown occurred and she got dizzy / passed out.
b) USSS could see she might do harm, so they hit her with a non-lethal weapon to induce dizziness or nausea. She's escorted off and no lasting harm is done to anyone.