Because he said something very stupid on television - indicating he didn't know what was going on at the convention center while the situation was being shown on national TV. And because everyone on site was reporting that FEMA was slow and incompetent.
Yes, that's my take too -- it was Brown's broadcast statements -- ignorance about the refugees and horrenduous situation at the convention center that did him in by providing the MSM a mega-Zot to throw against him.