Milk often boils over because of a complex interaction between heat and the milk's own composition; it's not simply a case of it "boiling and steaming."
Initially, as milk heats up, a very thin layer of fats and proteins (especially casein) forms on its surface. This layer appears as a thin film or covering over the milk's surface.
This film becomes a significant problem: it traps the steam that forms at the bottom when the milk reaches boiling point. Instead of easily rising to the surface and being released, the steam remains trapped beneath this layer.
