Greetings James,
Eastern white pine (Pinus strobus) routinely shed/cast needles as part of tree maintenance (needle senescence commonly witnessed in fall and winter). Needles are typically shed at the end of the second and third growing season for that particular year's growth. This problem is further noticed when the pine trees begin to mature and form a canopy, thereby reducing direct sunlight to the lower branches.
Needle cast is fairly common to established eastern white pine plantations and shouldn't be a cause for alarm assuming that you are not witnessing significant mortality.
However, if you are witnessing mortality, there may be several underlying causes: needle blights, insects (ex: Zimmerman Pine Moth), or root disease; and abiotic factors such as over-stocking, ozone injury, winter injury, high soil pH, and heavy clay soils.
Over the years, we have witnessed general eastern white pine decline in numerous plantations. The culprit, I suspect, is a factor of three aforementioned variables: over-stocked stands of pine, high soil pH, and planting white pine on heavy textured soils.
