the (natural) root zone of any large and tall tropical tree should usually be twice the size of the canopy. it is nature's way of keeping the tree balanced and upright in order to grow taller to reach sunlight in a dense jungle. in sg's urban environment, these trees pose a problem when they age as they grow taller, has a larger canopy, but root zone is artificially confined because of construction and concrete barriers deep in the ground where roots should healthily grow to twice the size of the canopy. in the orchard road area, trees are cropped to reduce their canopy size but such manmade measures don't deter the nature of trees to still require a large healthy root base as they age. other problems may be root diseases and root rot that cause root base to shrink and lose their strength in holding on to soil. another problem may be just a lack of soil deep and large enough for root to grow and get nutrients. i expect more trees to cum down on roadsides as some are reaching 69 years old with all the issues mentioned above for their root health.