You and Donut were following Needle's lead down the stairs while the zombies were following. Coiny had just been infected, and Pin had looked back to him.

📌 — ★°.· "Guys, we can still save hi—"

Needle then picks Pin up, and now both you, Needle, and Donut are running down the stairs, and Needle carrying Pin.

🪡 — ★°.· "Pin, NO! Coiny is NOT COINY RIGHT NOW! You'll thank me later, hold on tight!"

📌 — ★°.· "NO! NO!"

Zombie Puffball then flies down, and breaks the staircase, causing you and everyone else to fall into the sewers' water, including the zombies.