This is the base of a very large tree that fell over in the middle of the forest. The trunk tree on the other side is very large and mostly intact. Most center-forest trees do not fall over like this -- they die in stages and fall apart in stages.
My bet is that this tree pulled out due to very soft and wet ground combined with a massive snow and ice load. The massive rock that pulled up with the roots may also have prevented the tree from putting down a stabilizing root system.