Needle Slough (Picture here) is a natural process for conifer trees (pine, spruce, fir, or juniper). Each species of tree will keep its needles for a defined length of time and is a natural part of this process. Conifers are not true evergreens and must shed their needles to stay healthy. Some years, the needles will yellow and drop unnoticed by the homeowner. In other years, a large number of needles yellow at the same time in late summer or early fall, making for a striking display. Because the condition is triggered by the weather and other stress factors, many evergreens are likely to show symptoms at the same time. While the trees look unhealthy there is no need for concern. It will only last a short while.
That being said, the yellowing of needles should only occur on the inner portions of branches and not on the tips. Yellowing at the tips may indicate the tree is suffering from sun scald, freeze damage or a possible pest attack.
This is a natural process and usually only persists for a month or two.