You should check out his response to this -- it will explain to you clearly why he was indignant, and why he responded the way he did.
The reporter had just called his mother a racist, before the reporter asked THIS question.
Please understand, I fully understand why he was angry; I would have been upset too. But it's not always necessary to show the anger. Sometimes a more effective point can be made by smoothly slipping a verbal knife between the questioner's ribs.