Why is Cato whimpering?

I picked up a rawhide bone formed into the shape of a bone. I knew he’d make short work of it.
He’s been working on it diligently all afternoon.
Suddenly I heard him whimpering and called him over. He had chewed the top part of the bone of…and was holding it in his teeth whimpering.
I gently removed it from his mouth and looked for some sort of injury or blood. Nothing. But he quit whimpering.
I gave it back and started all over. :[ But he went right back to it with gusto.
It seems like I remembering the same thing happening to Dakota.
He’s fine now, apparently.
Any idea?