If by the first rebellion, you mean when the animals rebelled against Farmer Jones, the cause was a build-up of years of mistreatment of the animals capped by an extended period of starvation where Jones preferred to get drunk and be lazy to feeding his animals.
Old Major (who symbolizes Karl Marx) planted the seed of rebellion a good deal of time earlier when he presented his "dream" of a world where all animals were equal to the other animals. In his speech, Old Major chronicled how the humans didn't produce, yet took the fruits of the animal labor. He also reminded the animals of the cruelty of man that he would steal the animal's children and slaughter the animals when they were too old to work for him anymore. The rebellion song, "Beasts of England" riled the animals up to think revolt...which they finally did on that fateful night when Jones refused to feed them.
So "what it was" about was the animals banding together, deciding that they no longer wanted to be controlled by an outside entity...and rising up and defeating the current government...later replacing it with their form of citzens-as-equals government called "animalism".