Explain what of why this event had happened

At last the windmill is finished, with walls twice as thick as before. The animals are very proud of their achievement. Napoleon names the windmill “Napoleon Mill.” Two days later, Napoleon calls a meeting to announce that he has sold the timber to Frederick, not Pilkington. He denounces Foxwood and makes the pigeons change their slogan to “Death to Pilkington.” Napoleon claims that Frederick had never planned to invade Animal Farm and that he was not as cruel as rumored. Moreover, Snowball has never been at Foxwood or been Frederick’s collaborator; in reality, he has been Pilkington’s longtime collaborator. The pigs are proud of Napoleon’s shrewdness. They believe Napoleon’s claim that his relationship with Pilkington was just a pretense to get Frederick to raise his bid. Even cleverer, Napoleon refused to let Frederick pay for the timber with a check, instead demanding cash that he will use to buy the windmill machinery. Napoleon goes so far as to hold a special meeting where the animals can inspect the banknotes.

Three days later, Whymper informs Napoleon that the banknotes are forgeries. Napoleon sentences Frederick to death by boiling alive and tries to reconcile with Pilkington. The next morning, Frederick and his armed men overtake the farm. Napoleon considers calling Pilkington for help, but Pilkington sends a note that says, “Serves you right.” As the animals watch helplessly, Frederick and his men blow up the windmill.