It sounds like you opened a return request, and then didn't follow through with it - in those cases, ebay will always remove any feedback that is left. 

When a case closes due to the buyer not following through with the return, ebay sends out an email to the seller stating that all defects and feedback related to the transaction are removable because the buyer did not comply with the policies of the return system (i.e. you didn't return something you requested to return - I get that in some cases it would have been too much trouble, but to ebay it's just another buyer who didn't want to return an item they claimed was faulty).