By mauve I guess that you mean the vaguely market stall-shaped icon that represents https://www.openstreetmap.org/way/440864851 ? If so, it’s because it’s tagged “amenity=marketplace” https://wiki.openstreetmap.org/wiki/Tag:amenity=marketplace .

The actual bit of code that does this, if you’re interested, is https://github.com/gravitystorm/openstreetmap-carto/blob/f4919fff617490c6beac5a71f086d1088739b947/amenity-points.mss#L965 .