In Chapter Two, the animals rebel as a result of Jones' neglect. In sum, Jones has "fallen on evil days": he spends days "lounging in his Windsor chair," drinking alcohol and reading the newspapers. His men are just as lazy: they neglect the weeding and general maintenance of the farm and do not bother to feed the starving animals.
On the day in question, Mr Jones spent the morning in the pub and then came home and fell asleep while his men went out rabbiting. The important point is that neither Jones nor his men bothered to feed the animals, prompting a cow to break down the door of the store-shed in a frantic search for food.
It is worth linking these points to Boxer's famous maxim: "I will work harder," because this maxim demonstrates an important point about his character. Boxer is so loyal to the regime that he is unable to grasp that working hard has nothing to do with the way he is treated. In other words, the animals are mistreated by Jones because Jones prioritizes alcohol and sleeping above his farm. Later, when Napoleon takes power, Boxer continues to say this maxim but, once again, cannot see that working hard will not improve his life. Napoleon is a dictatorial ruler who keeps the animals in a state of oppression to ensure that his power and authority go unquestioned. Ultimately, it does not matter how hard Boxer works; he will always be oppressed and this is the story's tragic irony.