If you know the geography, demographics, and recent history, it's not at all surprising that someone broke through the protest, in the west-bound lane. It was early evening, so you have a lot of people (most of them white), trying to get back home to the suburban/rural white-flight zone of St. Charles County, Warren County, etc. And just as they're about to escape North County (home to Ferguson), they're detained by a Black Lives (and only black lives) Matter group? Well, shucks, how do you think they'll feel about that?
Of course, that's not my situation. I live in North County, with black and brown neighbors on all sides.