1. They've been looking for a good reason (or any) to move money away from them.
2. They really like you personally and want to do business with you, and peel off a small chunk.
In the second situation, you have to be appreciative of what they give you and not ask for too much. They will generally be a good source of referrals.
I had rich client like that, and I was angry that her rich ass only gave me a little bit to work with, but I kept my cool and she's sent me two excellent referrals in the last six months.