To understand why Julia rebels against the Party, take a look at Part Two, Chapter Two, when she meets with Winston in the woods. Despite being a member of the Junior Anti-Sex League, Julia claims to have slept with many Party members, including those in the Inner Party. For Julia, therefore, her rebellion is driven by a need to express herself sexually and to satisfy her own sexual desires. She does not want to be constricted by the Party's repressive moral code: she wants the freedom to be with whomever she chooses.
In addition, Julia rebels because she wants to have a "good time," as she explains to Winston in the next chapter. She hates the numerous rules imposed on people by the Party because they are designed to prevent a general enjoyment of life. In contrast to Winston's, Julia's rebellion is not based on Party doctrine or the intellectual dimensions of control; she simply wants the freedom to live as she chooses.