Yews are fairly easy to grow but can be affected by too little or too much water. And winter injury from bright sunshine and cold winds can cause the foliage of these evergreens to lose moisture when the roots are frozen. I am including a link to a web site with more information about common problems with yews.
I would also suggest that if you haven't done so already, you may want to get the soil tested in the area where the yews are growing which will give you a better idea of the health of the soil. Once you know that, it will affect how you water and apply fertilizer (which will vary depending upon the type of soil and organic matter in it). There is an easy soil test kit that can be purchased from MSU at the web site below.
The good thing about yews is that if the die back is not too extensive, you can prune it out and the shrub should send out new shoots next spring. So the healthier you make the growing conditions, the better chance you can save the remaining yews.
Hope that helps.