You may never know. In 1988 I was living and working in Cicero, NY, just outside of Syracuse. A co-worker of mine just had bought a resale home. About month living there one evening he heard a big thump that shook the ground. Looked outside and in his rear yard a tree had fallen. maybe 12 inches wide or more. Looking it over the trunk of the tree was cut almost all he way through at the ground level. Enough was left to hold the tree upright for who knows how long. Someone previous probably broke or jammed the chain saw and could not finish the fall. He was an extremely lucky that no one, especially his kids, were not in the yard at the time.