What we have here is something called natural needle drop. This is the ponderosa pine that blossomed back in the spring and had magenta colored cones - also a natural process every couple years.
Pine trees drop needles every fall. Ponderosa's keep needles for about 4 yrs - these needles you see on the ground first appeared on the tree 2005-06. If the needle drop is more than usual, it means that 4 yrs ago was a great year for growing needles.
Sometimes needle drop happens over several weeks, or it can happen much more quickly. They say it depends on the weather. Last Sunday, I was mowing the lawn and there was no needle drop. Today, voila!