Some possible explanations are that the alien species may be
i. Vigorously growing and compete with the natural plants for minerals, water etc. The less vigorous local species may be eliminated.
ii. Natural pests and predators of the alien species may not be present in the introduced area-leading to proliferation in their number.
iii. The introduced species may harm the local species by production of chemicals (Amensalism)
iv. The alien species by proliferation may make conditions unfavourable for the growth of local native plants. (eg. Eichornia)
