Getting to the root of the issue of why most trees come down naturally, you find, it's well, the roots. All puns aside, we all know how important the root system is to a tree. From Lateral Roots to Fibrous Roots (smaller masses of roots) to the main Taproot (which isn't prominent in every species), we are going to shed a little light on the part of the tree that typically grows in the dark.
Before we even have a tree, the soil and its consistency is important. The soil will determine how easy it will be to grow more roots in and the efficiency to gather nutrients. If the soil is crowded, compacted, dry, excessively wet, rocky, this will impact the development of the root system and in turn the overall growth of the tree.
The taproot is often the first main root that grows from a seedling. It grows downwards into the earth. We already discussed how the soil will affect the growth in this process, so let's say conditions are perfect for root growth. Not every tree will have a taproot survive long, be distinguishable, or even be very important long. For some trees, the taproot can grow upwards to 30 feet into the soil while other species taproots may go only 2 feet deep.
This is where the real support is, especially with the trees without a taproot. These roots will often far exceed the "drip line," the farthest out the branches grow. These roots are often thick and hearty. Depending on the species of the tree, they could grow up to 20 feet deep but are most commonly found in depths of about 2 feet. They can extend as far as 35 feet in all directions. Having a shallow support system, if the roots are not healthy or strong, then they can become victims of erosion or overbearing elements creating rot. And a rotting root system will cause the tree to topple.
These will grow downwards off the lateral roots. Even the best support systems need a little support sometimes. These will grow deep, as deep as the taproot (if it is there) or deeper if the species has no taproot. Soil consistency is another large determining factor for the growth of these smaller roots.
This small cluster of roots are found all throughout the root system, very close to the surface or even protruding out. These act as a highway or streamline system for oxygen and other nutrients. There is no major harm if these roots are exposed to the elements, other than possibly being a trip hazard for unexpected feet-dragging people.
It is a common misconception that the roots just grow straight down. Though that is partially true, for the most part, when you look at a tree, imagine it laying on the ground. The tip of the tree is about where the roots have grown, in all directions around the trunk. Pretty cool.