From my reading, it's flow that's causing the erosion. We've punctured the dam.
Consider an earth dam holding in a lake. As long as the water's sitting behind the dam, not much is going to happen but as soon as a little crack appears or a bit of water begins pouring over the top, it starts eroding - carrying away bits of the dam. As the hole gets bigger, the flow increases which accelerates the erosion.
Now picture the oil eroding a hole in the casing and surrounding rock/silt/etc.
Consider an earth dam holding in a lake. As long as the water's sitting behind the dam, not much is going to happen but as soon as a little crack appears or a bit of water begins pouring over the top, it starts eroding - carrying away bits of the dam. As the hole gets bigger, the flow increases which accelerates the erosion.
Now picture the oil eroding a hole in the casing and surrounding rock/silt/etc.
Here's a visual from a recent natural disaster: http://www.boston.com/bigpicture/2010/06/landslide_lake_in_p...