i'm not necessarily a cuck but i am a weird fetishist. so i think i have room to speak.
i imagine the psychology is actually similar to the reason a lot of women have rape kinks/certain people may be into ageplay/etc. it all stems back to wanting control over a certain event. i was raped when i was young, and now i have an insanely large rape kink. i'd say a lot of that stems down to wanting the trauma to happen again on my own terms in order to process it.
i think in the same vein, cheating is also very traumatic (or at least it definitely can be). in a sense, you could argue that cucking is a similar concept in which you'd want to take control over the event.