There are several reasons as to why a tree might lean. Normally, it is a combination of some/all of the points below:
- Weight distribution: Depending on the diameter of the trunk and especially the height of a tree, gravity can create problems. So the main question is how the weight of the crown is distributed over the tree's trunk.
- Species variability: Different species of trees have different root systems, some go deeper and others don't. The depth and width of the root system are factors to consider when assessing a leaning tree.
- Environmental factors: There are a variety of environmental factors that will contribute to a tree leaning. For example, a
tree that leans because it has grown towards the sun often has a curved trunk. The trunk curves due to the tree's ability to adapt over time to the changing
availability of sunlight. Its root system will grow to accommodate the “off center” weight distribution, up to a point. Additionally, storms or constant wind can obviously cause a tree to lean.
- Structural damage: Leaning trees that don’t have a “sweep” (caused by wind) but tend to have a fairly straight trunk generally have a history
of structural damage to the root system or storm damage that has
caused the root system to slip, break, sink or simply fail to support the
tree in some fashion. If the trunk appears to enter the ground without
a mound of soil on the side away from the lean, then the root system
has sunk, either from rot, mechanical fracture, physical cutting of the root plate or soil subsidence. If the mound does exist on the back side of the lean, that's a sign that the root plate is being tipped
out of the ground and the roots have failed - either through mechanical wounding or structural failure. As roots break under stress from wind or off center crown weight pushing the trunk away from the soil holding the roots, the up-wind side of the root plate will tip out of the ground.
In addition, it may be worth mentioning that time plays an important role here too. Older trees will have been exposed to the forces of nature and plant pathogens for longer than younger ones so it certainly makes sense to take the life cycle of a tree into consideration, some live longer than others and thus may have been subject to wind while the other surrounding trees weren't "tall" enough to shield off any of the wind etc.
Sources: Georgia Forestry Commission - Leaning Trees - What's up with that