It would seem from the wikipedia article that the piss and shit of Dave Matthews and his Band had amalgamated in the 800lb-capacity septic tank to form a uniform yellow-brown fluid, and that it was this shiquid that rained down through the grated surface of the bridge onto the unfortunate sightseers. Perhaps this liquification-by-intermingling explains the conundrum, or perhaps the band’s diet was lacking in fiber. I personally cannot attest to the exact viscosities of the turds as they left the band members’ anuses, but what we know for a fact is that at some point post-production, pre-projection over the Chicago River, the excrement lost its integrity.