Plugin: Cancel Feedback
Enable asking the customer reason for cancellation before processing an actual cancellation.
You can use placeholders %cancel_reason% and %cancel_comment% in cancellation email to admin.
Cancellation reason can also be found in Invoice Details in user profile and on special page aMember CP -> Reports -> Cancel Feedback
Also this plugin give you special reports that show distribution between cancel reasons within time period. You can run this report at aMember CP -> Reports -> Reports (Cancel Feedback)