I am not certain that Moses "decides" to return to Animal Farm. He simply reappears. The two times that Moses' presence is important are both when the animals on the farm are suffering incredible hardships. The first time he is present is when Jones is controlling the farm, exploiting the animals' labor and making them suffer in excrutiating situations. It is here that Moses starts on about "Sugarcandy Mountain," a vision of a world that is fundamentally different than Manor Farm. When the animals take over the farm, Moses' role is a bit less important because the focus of the animals is on the element of empowerment they feel in being in control of their own farm and what that represents to them. Moses appears at the end of the narrative, though. His appearance is coinciding with the difficult times the animals face under the pigs' rule. Orwell implies that just like Moses was Jones' spy, Moses is deployed by the pigs and those in the position of power to make sure that everyone on the farm can have their thoughts diverted. In other words, his presence and his talks about "Sugarcandy Mountain" are used to keep the animals' minds off of how their exploiters are no longer human beings, but rather their fellow animals in the pigs. The return of Moses indicates that while those in the position of power might have changed, the suffering of the animals keeps going with a steady cadence.