I checked out the handy-dandy Citizen ICam, the Chicago Police Department's database of reported crime, about the police action I mentioned a few weeks ago. Apparently it was a case of "Property (Domestic) -- Criminal Damage", which, according to their definitions page, entails:

Knowingly or recklessly damaging another's property, setting a fire on another's property, injuring another's domestic animal, and/or setting a stink bomb or other offensive-smelling compound on another's property.

Apparently there were firetrucks, so I'm guessing it was a spot of unauthorized flame-throwing or something. Fireworks gone awry? Possible. There seemed to be too many police officers there for it to be a stink bomb or something. At any rate, I haven't noticed any more massive gatherings of law enforcement officers, so I'm guessing it wasn't too major.