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The answer to is  convective currents or convection and expansion of gases when heated.

When milk is heated from below, the hotter milk particles rise to the top of the liquid because they become lighter. Then the milk which is colder on top due to its higher density goes down.  This form of heating liquids is called convection.

   When boiling point
temperature of milk (at the atmospheric pressure) is reached the water particles in milk take heat from the neighbouring molecules and escape in to the air.  So the cooled milk particles from near the open surface go down to the bottom.  This process appears very vigorously ebullient at the boiling point temperature.

Do you mean the overflow of milk over from the top of the utensil to outside over the rim?

Milk contains organic substances, and fat.  Certain dissolved substances in milk come out as gas on heating of milk. The gases coming out from inside the milk lift the creamy layer on top of the milk air boundary. The creamy layer on top of the milk is formed by the fats inside the milk.  The gases coming out of milk expand due to heat and so lift the creamy layer.  Milk bubbles formed due to surface tension are able to rise to the brim of the utensil.

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