I think they were expecting the containment chamber to hold the blast. Either they had too much fireworks crammed in there for what the chamber was designed to withstand...or those illegal fireworks were loaded with a crap ton of black powder.
One of the most popular fireworks in this area is the mortar shell. Lifting charge and then the dispersal/display charge as a payload. The bigger the mortar, the more spectacular the display, but those lifting charges are nothing to screw with. If there were say a ton of those in the batch, the 'planed detonation' should have been done in a hole in a gravel pit from a distance, not on a city street.
I'm no EOD guy, just a li'l bit country bumpkin that likes the occasional fun loud noise, but I think they grossly underestimated the power of what they had on board.
Just hitting it all with a fire hose would have been much smarter than what they did, even if a soggy mess isn't newsworthy.