So, you want to know who defriended you or removed you from a Circle on Google+? Are you sure you want to know?
Google- (Google Minus) is an application that monitors the circles you are in and notifies you when you are removed. It's a shame it doesn't come with a reason, or perhaps its a blessing that you don't have to enter a reason when removing someone from a circle.
Get Google- http://apps.kynetx.com/installable_apps/4549-GoogleMinus