Per Buck Harvey's Twitter:
Sources say James Anderson's agent this month encouraged Spurs to trade his client. Nothing offered would have been better than JA Monday.
Note: Anderson will be an unrestricted free agent on July 1 after the Spurs declined their 2012-13 team option. He will likely leave San Antonio in the summer and sign with another team, so there's a chance the Spurs will trade him before he leaves for nothing.