The issue actually starts with Alice, as she sends Tx1 with a very low fee. However, Carol (the attacker) exploits this situation further by creating Tx2, a large child transaction with an extremely low fee, which brings down the fee rate of the whole package.

Without Carol's interference, Alice’s Tx1 might get confirmed if someone uses a CPFP method.

In short, Carol’s actions as the attacker are what turn this into a pinning attack.