What happened?

Can someone explain me what just happened? Why is "my" monster returning to my opponent side?

Crackdown doesn’t say that the stolen monster is returning to its original side of the field if it (Crackdown) destroyed or left the field?

Is it some basics Yu-Gi-Oh rule? Should something like « take control of that monster (even if this card leaves the field » should have been wrote on the card so I would keep the monster?

(I’m just a casual player, I don’t know all the ruling, I try to improve myself every time)

Thanks for the feedback! 🙌