Since quite a few are asking, I figured I'd provide the pertinent sections of TFAs.
According to an update in the Boing Boing article, Watts got out of the car to ask what was happening -- presumably because his car and/or person was being searched. When the officers refused to answer and told him to get back in the car, he asked the question again. At which point he was attacked, his property was seized, and he was asked to waive his Miranda rights.
Sounds like the unfortunate combination of a pissed off officer and a less-that-sympathetic citizen compounded by detectives/officers who get pissed when prisoners refuse to talk. I can empathize with both parties (first and second, not third -- right to remain silent means right to remain silent,) but -- assuming the accuracy of Watts' story -- the assault charge is probably trumped up. Convincing a judge of that is a whole different story.