Why does the nucleophilic addition of a cyanide ion to an aldehyde form a racemic mixture?

Because the plane of the C=O bond in an aldehyde is flat, and so the nucleophilic attack of CN^- onto the C can occur above or below the plane of the molecule, producing two different stereoisomers of a 2-hydroxyalkane molecule with 50/50 split (as both structures are equally thermodynamically favourable). This produces a racemic mixture