I would say that the peroxide is also reacting with some of the odour chemicals. Peroxide is used for disinfection purposes because it is so reactive. It kills off tastes in water. The shampoo is acting as a surfactant to increase absorption of the material. An example is how it helps to break down grease. It makes oil soluble in water (even skunk oil). Then the baking soda and peroxide can chemically react with it.

At least that's my guess.

Dan O.