Bit late on the reply for this one, but theres some Archives the elaborate on A2's backstory. I missed them too, so I had to go read up on it in the NeiR wikia.
She was part of an early YoRHa unit who's mission went wrong, she was the only survivor. She became a recluse after that, blaming both YoRHa and the machines for what happened to her friends, and vowed to wipe out all machine lifeforms. Anemone was also involved in the mission, which is how the two know each other.
A2 and her unit ended up being a test that was meant to fail, in order to continue perpetuating the machine/android conflict. The test also resulted in the creation of both B and E units, which is why Anemone mistakes 2B for A2 the first time you meet her in the game.
She had never encountered Pascal before her route, so she had never really seen pacifist machines. I think being forced to interact with Pascal because of Anemone, paired with receiving 2B's memories resulted in A2 becoming more social.