Hello, it is helpful to get more information. Do you know specifically what species of tree you have? You mentioned them being pine (based on needles, it may be a spruce). How many trees do you have, and how many are showing symptoms of needle drop. How long ago did you plant them? How soon after that did symptoms develop? MSU has developed a bulletin on top do's and dont's of tree planting: https://www.canr.msu.edu/resources/christmas-tree-planting-guide, this is geared towards Christmas tree farms. When trees are first planted, they will undergo stress. Depending on a host of factors, that could be what has caused the needle drop you are seeing.

Thank you for your quick response!

Hi, from the information at hand, it is likely related to planting stress.  Hopefully the trees bounce back next year and will outgrow the problem.  Black hills spruce are susceptible to a few different diseases that can cause needle drop as well (Weir’s Cushion Rust of Spruces – Wisconsin Horticulture, Landscape: Stigmina Needle Cast | Center for Agriculture, Food, and the Environment at UMass Amherst )

If the trees show any repeat (or other signs of trouble) next year, you may need to consider sending in sample to diagnostic lab for formal testing (Plant & Pest Diagnostics).