Why does your lip quiver involuntarily when you begin to cry?

Question

Also, why only the lip in particular?

In:

Answer 1

I think the leading theory on emotional crying is that it’s a clamp-down on the fight-or-flight response. Your eyes tear up to impede you because from the DNA’s point of view, it’s safer, for women and children anyway, to stop running and fighting at some point. Hormones found in tears give some credence to this. I don’t know how lip quivering factors into it.