That means there are some green needles throughout the year, but the trees do not keep their needles forever. Most ponderosa pines will dispose of needles which are 3 or 4 years old. This can happen in the fall or the spring. These needles are shaded by the outer, newer, needles and do not receive enough light to photosynthize and produce food for the tree. Pine trees shaded by other trees will also exhibit needle thinning.
Some years there appears to be more brown needles than other years.
This is due to the fact that if needles from a good growing year are being dropped, there are many needles to drop. So, if the weather allowed good growth 4 years ago, then this year we can expect a lot of brown needles. This year was dry in the winter and early summer. This resulted in drying of the new needles. Winter drought causes the needles from the previous year to brown back from the tip.