In Bible times butler was produced by placing milk in a bag made form an animal skin (see notes on Psalm 119:83 and Matthew 9:17) and shaking it vigorously back an forth until the butter fat separated from the milk.       The proverb has a “just as” construction. Just as surely as shaking milk produces butter, giving vent to one’s anger will cause strife and trouble. The phrase “stirring up anger” suggests that a person can feed his anger until it becomes uncontrollable.

PROVERBS 30:33- The churning of milk bringeth forth butter. . . so the forcing of wrath [stirring up anger, NIV] bringeth forth strife.