In Animal Farm, what happens to Snowball during the meeting about the windmill? What events in Soviet history does this scene suggest?

He is suddenly declared a traitor and escaped from Animal Farm with Napoleon's snarling dogs (remember Bluebell's puppies?) hard on his heels. 

Snowball represents Léon Trotsky, a militant communist leader who opposed Stalin's regime. He was the founder of the Fourth International League (which contested Stalin's Third International League though never managing to overpower it.)

In the power struggle which ensued, Trotsky (or 'Trotski' or 'Trotzsky') was "chased" from the government in 1924, then excluded from the Communist Party in 1927. He was banished from the USSR four years later after being sent into exile in central Asia. He was ultimately assassiated in Stalin's political purge.