If you look at the shape of the eye brows
you can see the lines like two mountain curves.
This picture from Pentaxuser is called "frowning mountains":
You can see the same shape like frowning eyebrows which I have indicated with red lines.
I have found the phrase "frown on/upon" explained as
disapprove of, dislike, discourage, take a dim view of, look askance
at, discountenance, view with disfavour, not take kindly to, show
disapproval or displeasure (Free Dictionary)
Frown down is more difficult to find. In the description of frown, I have found this:
To look or act disapprovingly or threateningly; lower: as, to frown
upon a scheme. To repress or repel by an aspect of displeasure; rebuke
by a stern or angry look or by severe words or conduct: as, to frown
one into silence; to frown down a proposition. (Wordnik)
Now a mountain is tall and looks like frowning (that's why it is said to frown down). Depending on the context, it can mean either intimidating or threatening, hostile.