Now, why are they pushing?
Because they are stacked and in RESISTANCE to each other.
Let's look again at what you stated earlier...
what is pushing the topmost layer of air molecules down on the molecules below?
Basically, nothing.
Imagine you are the last molecule and you're laid on your bed face down. Your body density is being resisted by the mattress and then the bed frame and then the floor and then the joists. Your back and everything facing up has no force upon it so it has nothing to push against. It becomes dormant. It freezes against a true vacuum (possibly) but your front is still resting on the mattress molecule which in turn is resting on the bed molecule which is resting on the floor molecule, etc, etc, etc.
Your back
If I line up 10 small blocks, all touching, which end is under the greatest pressure if all are at rest and not pushing or being pushed one way or the other? If one end is not pushing at the next one in line, then that next one is not pushing against the one beyond it.
The pressure will equalize if nothing is pushing each individual molecule all in one direction.
You said nothing is pushing the molecules, so why is there higher pressure at one end than the other?