Milk fat as it occurs in milk has unusual characteristics. It is made up of fat globules which are 4-10 microns in diameter. These globules are covered by protective membranes made of bipolar compounds phospholipids, protein, diacylglycerides and monoacylglycerides and other surface active materials. They are collectively designated the milk fat globule membrane (MFGM).
It is likely that the violent mixing of milk, during churning, disrupts a sufficient portion of the milk fat globule membrane (MFGM) to render milk fat globules ‘sticky’.
Further churning simply cause the ‘sticky’ fat globules to aggregrate, form bigger butter granules and rise rapidly. Excessive churning increase abrasion and leads to quicker separation.