I don't deal with oaks very often and never dealt with a shingle oak. But I'm guessing it's one of the root rots, either ganoderma or inonotus. In either case, they are a root/butt rot which makes them very susceptible to toppling. I'm afraid that they are sign of a death sentence as it is a hazard if it is near any kind of target.
I will waffle by saying that I could be wrong with my ID and would need to confirm.