Once the pigs start walking on two legs ,two legs became better then four
Commandments had been rewritten and animals are worked way more then they be. Power made Napoleon evil. He took advantage of his power to have the animals work in his favor. The animals all feared Napoleon.
Animals no longer where equal on the farm. Pigs starting doing things human did. They walked on two legs along with sleeping on beds. One time the pigs also got super drunk.
Animals soon could not tell the difference between pigs and humans. Pigs stayed ruling the farm and taking charge of younger animals. As you can see after old majors death Animal Farm went down hill. Pigs bring up the "rebellion" to younger animals.