Well, luckily we managed to find some kind of logic in this particular case but there are a lot issues that can seem illogical to you like:

Забыться (тяжёлым) сном
Идти лесом / горами
Пробираться кустами
Оказаться умным парнем
Родиться крепким ребёнком,
and so on.

I think even native Russians can't explain the reason why the instrumental case is used for these and like. So I just suggest that you simply memorize a verb along with the case it needs, not wracking your brains around the reason why, just like I already mentioned in my post #82. I think it's going to spare you a lot of effort and brain cells