In the Tang Dynasty, the buffer region was very powerful. Officers in the buffer region have local military and political power, which can be inherited and do not listen to the central command. This is mainly due to the disintegration of the military system of the late Tang government and the adoption of the "recruitment system." The army is under the command of local military officers, with heavy external forces and light internal forces, resulting in weak central power and inability to control the localities. Coupled with the political corruption of the central government, it lost its ability to control the whole country. Local governments also have economic autonomy, which eventually led to the formation of separatist regimes in buffer areas.
The second reason is the Huang Chao Uprising.
The Huang Chao Uprising greatly accelerated the demise of the Tang Dynasty. It further dealt a blow to the strength of the central government and the influence of local families, and made a number of new warlords rise one after another under the banner of suppressing Huang Chao and the king of Qin. Since then, local separatist forces have formed. It's a bit like the situation in the late Eastern Han Dynasty, where the whole people engaged in local separatism under the banner of safeguarding the central government.
In 907, Zhu Wen deposed Tang Aidi, the last emperor of the Tang Dynasty, and established the "Liang" regime, known as "Hou Liang" in history. The Five Dynasties and Ten Kingdoms officially began.