I'm getting out of my comfort zone talking about fall needle shed. I don't think we have a good explanation as to why it is happening. It is certainly associated with stress and is worse in some years (like this one) and in some fields than others.
Of course some needle shed occurs every fall. Usually it's the 4th or 5th year needles. The problem occurs when it's the 3rd, 2nd or even the current year's needles. That's when tags start coming out of trees!
This tree had only a few areas that had discolored needles.
I've had some calls from growers that are concerned that scales are causing this discoloration. At this site, there were no scales. If in doubt, look at the shoots of several trees. Turn the foliage over to look for the presence of scales. Most of the fall needle problems have not been associated with scales.
Advice for the grower? Keep on top of your fertility. By fertilizing according to soil sample results, hopefully you can avoid many of these problems. If the problem develops, take soil samples and plant tissue samples of healthy verses trees with shedding needles. And get your County Extension Agent involved.