Trees growing close together or near buildings often lean - sometimes dramatically - in the direction of bright sunlight, indicated by arrow at right. This doesn't happen because the trees are growing "toward" the sun, but rather because they're simply growing faster - adding more leaves, stems and branches - where there's sufficient light to support new growth.
In other situations, half of the root system of a tree may have been lost during a construction project or may simply be poorly developed due to obstructions (large rocks, compacted soil, street curbing, etc.).
Or, improper pruning or storm damage may result in the loss of all of the branches on one side of a tree, leaving it lopsided.
Regardless of the cause, leaning and lopsided trees become more likely to fail as they grow larger and more unbalanced over time.