According to Old Major, what is the cause of all the animal's problems?
This is a good question. The speech that Old Major delivers in chapter one is absolutely clear on the root cause of the problems of the animals. For Old Major, the problems of the animals can be summed up in one word, man. Here is a quote that shows this point very clearly.
Because nearly the whole of the produce of our labour is stolen from us by human beings. There, comrades, is the answer to all our problems. It is summed up in a single word — Man. Man is the only real enemy we have. Remove Man from the scene, and the root cause of hunger and overwork is abolished for ever.
Later on Old Major even makes the remark that man is the only animal that consumes without producing. In his mind, man not only does not work, but he also uses the animals. More to the point, he uses the animals until they lose their usefulness. After this, he either will kill the animal to consume its flesh or he will sell the animal to a slaughter house to be turned into something else. Hence, according to Old Major, the lot of the animals is nothing but labor and then death. And all of this is under the hands of men.