Cambodia secretly encroached on Thai territory by using civilians to invade and settle there while Thai soldiers were asleep and not patrolling. After a while, they became active, claiming the land as Cambodian. Thai soldiers eventually drove them out.
It's worth remembering that Hun Sen has been teaching Cambodian children false history, claiming that Thailand stole land from him, using fairy tales in kindergarten, thus creating hatred towards Thailand. Hun Sen is a jinx to Cambodia.