Sean ... In answer to how many .... very few (<1%)...
The one pictured in my post (#4) was the most dramatic I've seen (inspected) with regard to the size of the bridge.
This tree (Forest Red Gum) is now gone ... A TMO consented removal ... & not of my doing.
It was located in a suburban backyard: a remant tree of the former Turpentine/Blackbutt forest in this area (but much less represented in this precinct). I missed one photo in my earlier post....
As to the etiology of the phenomenon in your pics... I have no answers ...