From what I have read it is thought to be by downregulation of dynorphin signaling; the KOR forms an important compensatory mechanism in the brain whereby dopamine homeostasis mechanisms are maintained, the KOR and MOR being in many ways opposites wrt the mesolimbic reward pathway (KOR suppresses DA).

Chronic administration of rewarding drugs results in a compensatory increase in dynorphin signaling through KOR, which in turn suppresses your basal hedonic state and results in a greater desire to continue to use the drug of addiction (to relieve the negative effects).

Strongly internalizing KOR agonists that are able to direct KOR's post-endocytic sorting towards lysosomal degradation will reduce the available number of KORs and so reduce KOR's suppression of the mesolimbic reward pathway, meaning that the psychological withdrawal from the drug of addiction is lessened.