Trees get weaker due to several reason . The main reason is pollution , pollution has degraded the quality of trees and as well as of environment . Due to increase in the pollution the level of carbon dioxide increases that causes air pollution . Air pollution not only effects on the environment but also causes decrease in the ozone layer that protects the environment.
Other reason is deforestation , means cutting down of trees on large scale this has totally affects the trees and thus they become weaker .
Soil erosion, soil binds up the trees and hold it but due to soil erosion the trees become weak and thus fall down in winds .
Hope you will be satisfied with my answer.