Here's a plastic bottle. Top bit is the cap, just below that is the loop.
Yes the piece that sealed it before you opened it.
When the bottles are crushed, the loop can come off, as we demonstrate here.
This loop can cause death to all kind
of creatures if they put their head or body
through it and it becomes caught.
So snip them before disposing of them, (no need to take them off, just hook scissors underneath), job done.