I think that these two leaving the farm shows two sides of the suppression of individuality. When Mollie leaves, we see suppression of personal choice. When Snowball is chased off, we see suppression of political liberties.
Mollie is vain and selfish and pretty unlikeable. But that should not be a crime. However, in the pigs' version of Animalism, there is no room for this kind of foolishness. Everyone is supposed to focus on work the way Boxer does, not on ribbons and silly things like Mollie does. In other words, you are not free to be the way you want to be.
When Napoleon chases Snowball off the farm, we see that you are not allowed to hold ideas that go against those of Napoleon. You are not allowed to be an individual in your political opinions.
In these ways, these two animals' stories show us two sides of the them of suppressing individuality.
In Ch. 5 we read of Mollie's disappearing act. It is reported in the following manner: "Three days later Mollie disappeared." Prior to this she had been behaving differently - coming late for work, complaining falsely of "mysterious pains," and admiring her own reflection in the pool. She was also noticed fraternizing with people in the neighboring farm owned by Mr. Pilikington. Mollie represents the dissenters in Communist Russia who fled Russia for the attractions of the Capitalist West.
Snowball, of course, represents Trotsky who was the main challenger to Stalin's [Napoleon's] authority. In the ensuing power struggle between these two charismatic leaders it was inevitable that one should be eliminated. In this case it was Snowball.
Mollie left on her own without the knowledge of the others. Snowball was forced to leave by Napoleon.