Quite frankly......no.
Deer ribs aren't tough objects so Ihave doubtsthat a center hit rib obliterated a broadhead by itself.
My guess (without being there obviously) would be a loose broadhead that seperated from the insert enough to shear it off by most likely a sideways force such as her muscular legs slamming into the arrow.
If the head was completely seated then the insert provides the support to prevent it from shearing below the level of the threads.....I would think it almost would have had to have been loose from the get go. That is the only way in mind that I could see the head snapping that low on impact.
I just can't see a straight on impact into a doe's rib bone snapping off the head at that level if it's seated properly?

How much penetration did you get? Is there a chance you hit the shoulder itself rather than the "sweet spot"?

Odd indeed, but I'll tell you this.......I just transferred 5 arrows from my bow mounted quiver to my Cat-Quiver in anticipation of tomorrow and Saturday's rainy weather and 3 of the 5 broadheads were more than a full turn (one was 2 turns) out. This backing out came as the result of just traveling with the bow and hauling it up and down the tree, and removing the quiver to hang in the tree since Saturday.........no shooting with it on and they still backed out.
It would be a very simple oversight to accidentally nock an arrow with a loose broadhead.

What model broadhead are we talking about?