Knife not working as expected on the cartridge cut, then move face

Hi there, been following along to this tutorial with zero problems until i got to the part where you use the Knife Tool to cut the shape of the cartridge out

I follow along, doing as you do, but when i come to switch to the side view to drag the face down, it pulls more than just that part, as though it is somehow still connected to the otehr part we just cut it from

Stuck here and cannot move on without fixing this, but i have zero clue what has happened to make mine different to how yours moved down fine