Why is my upright Juniperus Squamta turning brown?

This is usually a watering problem - perhaps one pot is in a windier spot or manages to get more sun - if its turning brown now, the problem will have started during the heatwave we had and you're only now seeing the result.

The sun position sounds good.
If they have been here for a few years, I doubt the problem is caused by draughts.
I would guess the problem is underwatering. You must regularly water container grown trees and shrubs. Feed half strength plant food every two to three weeks during the growing season.
Repotting and root pruning every couple of years will be beneficial.
Use your fingernail to peel back a piece of bark low down on the trunk, if its green the trees alive, if its not its doomed!
If its alive, do the same further up the trunk and say those glorious words, "It's alive! YES! its ALIIIIVE!