Hi,

You have more than one surface, it looks more like an open polysurface...I can't tell for sure from the screen shot, but it seems like you have a gap between the surfaces on the corner, which would cause the shelling to exaggerate that condition.
Also, if the shelling distance exceeds the overall proportions of the surfaces by too much, it will cause them to break apart. Oh.. one more thing :-) it just occurred to me that the shell could have gone the wrong direction. Did you try checking "Flip"?

Jesse