Yes, it is most likely that squirrels are nipping the branches. An internet search yields various references to this from around the country, with eye-witness and photos to prove it. I bet you could catch them in action if you spent some time looking, particularly in early morning. It squirrels do this for a variety of reasons, such as nest building, seeking moisture, seeking food (buds), or maybe even just for practice or boredom when they don’t actually do anything with the cuttings. Not much can be done to stop the problem, but the good news it that it generally does not affect tree health significantly. Although it seems alarming, a healthy tree can take a bit of twig loss.

If your trees are relatively isolated from other trees you can consider wrapping the trunks with aluminum flashing for at least 18 inches on the trunk. (If other trees are nearby, they will easily jump from the top of neighboring trees). Place the flashing at least 4 feet off the ground so they can’t jump over it. If you must use multiple sheets to wrap, be sure the overlap is always on the top over the sheet below, like shingles. Otherwise, squirrels will use the edge to climb. Also, you could trim branches up to at least 5 feet above the ground so that squirrels can’t leap up and access the tree.

Another thing that squirrels do is strip bark off of young maple branches, so watch out for that too.

Yes, those dern squirrels. As you noted in your article, the fir cones can pick up a lot of speed falling from 100 feet plus. I recall a camping trip in Shasta red fir country in September where the squirrels were cutting the large red fir cones and you could hear them sizzle through the mountain air. When they hit the forest floor it made a resounding BOOM that made me think at first there was someone shooting.