What we saw with Isabel is the breakdown of the eyewall into a number of mesovortices. They are pretty cool to watch, but not exactly the easiest things to model or understand dynamically.

Essentially, when the system gets wrapped up too tightly, the eyewall can breakdown into these vortices. Essentially, the eyewall is a sheet of what we call vorticity -- cyclonic spin. If the conditions are right, as they sometimes are with very intense storms, the eyewall can become barotropically unstable (what that exactly means is a matter which could take awhile to explain due to the math, but focus on the unstable part), leading to its spin-up into a number of vortices. This is where the mesovortices seen with Isabel arose from.