What triggers the boars or deers to go back?

I have had this question for a long time. Sometimes when I watch the pros play, I see them making the boar to freeze exactly at the TC and shoot it when the sheep vils finish eating the sheep. But when I try it, I couldn't get the boar stop at where I wanted it to. It goes back immediately after it loses track of my vil.

I guess the algorithm behind the scene is the same for deer. What triggers the animals to go back? Does anyone know?