Mine do that too, it is because they are chewing on the ends of the bones. I have never had any of my dogs break off chunks large enough for them to choke on. They are trying to get to the good stuff inside.
There's a fine line between genius and insanity. I have erased this line.
Ruby, to my surprise, eats an entire hockey-puck shaped soup/marrow bone in a sitting. So, fresh bones must definitely have a lot of "edible" in them not just the "chew" use.