According to the Global times and Moderndiplomacy, it is due to the legacy of French colonialism.
Short answer:
“Many of the issues are not due to deep-rooted animosity between Thailand and Cambodia themselves, but rather stem from the lingering problems left behind when French colonialists withdrew,” Song Zhongping, a Chinese military affairs expert, told the Global Times on Thursday.
Long answer -> https://moderndiplomacy.eu/2025/06/08/thai-cambodia-border-conflict-and-colonial-legacies/