I've ridden in Critical Mass at least 3 times, the largest I remember being the 10 year anniversary. It starts out a fairly happy pro-bicycle demonstration but it often gets ugly. Some drivers get irate at being delayed and retaliate by honking their horns, yelling abuse, driving into the procession and sometimes outright attacking cyclists. Many cyclists react by yelling back at any honking cars, blocking any vehicles that try to drive through and supporting any cyclist that is in danger of being assaulted. Critical Mass is a show of force, and while one cyclist can easily be abused or threatened by a car, hundreds cannot.
It is no surprise to me that a family in a minivan could end up having a scary run-in with Critical Mass. If you don't know what Critial Mass is and what not to do, then it would be easy enough to get cyclists riled up. It sounds like the family probably drove into the midst of the cyclists, may have inadvertently bumped a bike and tried to drive away from the bikes. Staying still and letting the cyclists pass is usually the best thing to do, but that doesn't excuse what the cyclists did. Smashing a window and threatening a family is the work of assholes, the kind of people that like to incite mob violence and avoid responsibility for their actions. These are the people that make me avoid participating in Critical Mass.