At the site of the collapse, it's easy to see that the masonry is saturated. You can also see the phases of construction of the wall. The bottom part, below a layer of concrete exposed by the collapse, was built between 270 and 280 AD by Aurelian (and by Probus, his successor, after Aurelian was assassinated), while the upper section was added about 30 years later by Maxentius in preparation for the arrival of his threatening rival, Constantine. (Max then made one of his millennium's most hare-brained military decisions: he left his then impregnable Rome to fight Constantine at Saxa Rubra, lost big time, and died running away while trying to squeeze his army back across the Milvian Bridge.)
The sequence of the wall's collapse and its underlying cause are fairly obvious. Everything below the exposed layer of concrete is waterlogged, and a wider section of the saturated part than of the drier masonry above it fell away. It's clear that the collapse started when the wet lower masonry crumbled and that then the sagging of the concrete layer brought down what was above. This scenario is born out by the position of large chunks of the upper wall on the top of the ruble heap overlaying small pieces of masonry from the lower section of the wall.
And that ain't all! The expert at the site of the collapse suggested that I should also take a close look at another part of the wall about 100 meters east of the collapsed section. It showed a similar pattern of saturation, and large sections of the outer brick surface of this part of the wall are already missing. (See the picture at the site given above.) He said that this section would probably be the next part of the wall to collapse.
There were a lot of "suits" in the area, some of them being interviewed in front of TV cameras. It was obvious that no decision had yet been made on what to do about the problem: the only work being done in the area was by a fencing crew, which was driving in anchors for a more permanent barrier to keep the curious away from the collapsed section and from adjacent sections of the wall.
For more information on Rome's walls,
go to http://www.mmdtkw.org/VWalls.html
P.S.: The area right up to the walls as well as the grassy area right across the street from the walls are newly mowed and squeaky clean. I wonder if maybe the "lawn-mower-man" might have knicked the wall in just the wrong place?
Update: In April 2002 the repairs to the fallen section of wall are almost complete. Other nearby sections are fenced, and there is scaffolding in front of some nearby areas on the outside face of the wall.