It looks like the droppable plugin doesn't uses the correct area that has to be droppable when a CSS transform rotate is used.
Look at this Fiddle example:
The droppable plugin is applied to element B. The draggable plugin is applied to element C. Element A has no function, it's only there to view the working drop area.
When you drag element C it will only highlight element B when you hover the area of element A. But when you do not drag element C, but hover with your mouse pointer it will highlight element B as it should.
Why is the droppable area incorrect?
I've tested in Firefox 21, Chrome 26 and IE9 and IE10. They all do the same.