The red pigment in strawberries is an anthocyanin. This kind of pigment changes color depending on the acid/base concentration of it's environment. It could be the acidity of the yoghurt that is causing the color change. It could also be a chemical reaction called oxidation that is causing the anthocyanin molecule to change in subtle ways which cause the color to be perceived as brown rather than red.
making a small donation to science.ca.