Hollow Hickory logs

[Andy1981]

 I have a section of my woods that has a lot of bitter nut hickory trees , they seem to be a variety of sizes from saplings up to a few that are over 30" with a majority being in the 20" range. I have noticed that a lot of them have a green moss on the trunks and a few that I have cut up , either uprooted blow downs or broken from wind damage seem to have hollow or rotted centers for the first part of the but log. I also noticed that some have a lot of flare at the stump almost like a cypress tree. Is this a normal characteristic or is there some kind of disease affecting the stand?