1. Loss of Water management
Less devotion of the people to the god-king led negative impact to the empire. The Khmer people seem reluctant to work passionately for the king as a holy service as they'd previously did. The previously efficient drainage system became silted up with less water supply and the rice crops, accustomed be cultivated 2 or 3 times a year, were dramatically dropped, so weaken the productivity and the strength of the Angkor Empire.
2. The Presents of Buddhism Theravada
The Angkor Civilization was supported by the belief of Hinduism and Mahayana Buddhism. The monarchs being thought to be the god-king were able to inspire the dedication of their people to serve the throne as a religious ceremony. The empire's extensive irrigation system and military troops required huge laborers and workforce to maintain. The introduction of Theravada Buddhism in 13th century to the Khmers had clad to harm sublimely the basic foundation of the Angkor Empire in the long run. Theravada Buddhism educated the people to abandon worldly things, seek self-enlightenment, and discourage any superstitious notion belief which means directly or indirectly all divinities and all evils. The power of the monarch as a god-king or deva-raja was primarily challenged.
3. External Threats
As bordering states of the Angkor raised, they became serious threats to the empire, mainly the Ayuthaya, Thai State in the West. In order to shield the empire, the Angkor had to direct portion of its workforce to secure robust soldiers that in turn, run-down itself from maintenance to its irrigation system.
4. Two-edged weapon system of Roads Network
The road network designed by Jayavarman VII had aided the transports of product and trades throughout the empire and also expedited the Khmer troops to quell its neighbors. It had become an ambiguous blade when the Angkor became weak as the invaders could easily trooped through this road, rather than previously sailing up from the Mekong River. This clad to be true when the newly emerged Ayuthaya, a Thai kingdom in the West developed sturdier. They used this road to head from the Chaophaya through Phnomrung (in Burirum of today’s Thailand) then march to Aranyapathet to attack the heart of Angkor and finally seized the empire in 1431. The glory of the Angkor Civilization was terminated since that time.