Group: current SVN version
1) If you assign a vertex group to an "explode modifier" you can't delete it or change your mind. The only solution is by deleting the explode modifier or to delete the vertex group. There is no menu entry "none".
2) If you have several vertex group in the "Links and Material panel" and one of these vertex group assigned to the explode modifier : if you delete the vertex group assigned to the explode modifier, the next vertex group in the list is automatically assigned to the explode modifier rather than leave a blank area (or none)
windows svn, revision 14742