What Really Happened

First, let's define the building as building, noting the stepped ledge structure at its top; the cement structure overhanging building to shed water away from building as cornice; the 30 tons of functioning counterweight wall that was removed from both the Harvard St and Chestnut St. cornice tops during summer due to crumbling as parapet. No building permit was noticed by this close observer during that time, but I certainly could have missed it.
Paste a closeup of the remaining cornice into powerpoint and draw the building and cornice in cross section with an approximation of the absent parapet.

Remove the picture leaving the cross section.

Remove the cross section of the cornice corner block to represent the majority of the cornice along the length of the building.

Assume a similar density in both the parapet and cornice for simplicity. Draw starlike rays from each corner to every other corner of the parapet/cornice structure resting on the building ledge to graphically represent the center of gravity using a circle. Draw an arrow straight down.
Result = center of gravity appears to be on the building when the parapet counterweight is present.

Remove the parapet from the model.

Redetermine the center of gravity using the starlike ray method described above.
Result = center of gravity appears to be over the sidewalk below. This is where the Harvard St. cornice fell and where the Chestnut St. cornice is headed.

Failure to replace the counterweight function of the crumbling parapet during the summer is the root cause of the falling cornice onto Harvard St. at approximately 6:15 pm on 11/22/05.