Why do we say 'knitting of the brows'?

I think it's 'cause of all those stitches in a knitted brow, Wordwind.

Every one of those stitches represents a mess of trouble. We stitch up something when it's coming apart, usually at the seams.

A knitted brow is nature's way of making amends.

Here's a good antidote for a knitted brow. It's the poor man's Botox:

Pack up your troubles in your old kit bag
and smile, smile, smile!
While you´ve got a lucifer to light your fag
Smile, boys, that´s the style!
What´s the use worrying?
It never was worth while
So, pack up your troubles in your old kit bag
and smile, smile, smile!

Pack up your troubles in your old kitbag
Composed by Felix Powell; Words by George Asaf

http://www.westfront.de/packup.htm

The other verses are fun as well. :)