I may be wrong, but it looks like your tree was the victim of root girdling. That is caused by the roots wrapping themselves around the trunk rather than spreading out/down to support it. Or, in the case of your tree, the roots did grow out on one side, but did not give the tree enough support to maintain it.

Root girdled trees often have bases described as looking like telephone poles; normal trees tend to flare out at the bottom.

I know it's tough to lose a good shade tree, I lost a big sugar maple during 'cane Isabel a few years ago, also due to root girdling. If you decide to replace it, ask the nursery staff to show you how to root prune a new tree to prevent girdling.

Some times people will leave the burlap, paper wrapping on a plant/tree and it starts the roots wrapping around instead of spreading out...That's a weird looking stump...

That is strange!

My ash tree is huge, but it is starting to get dead spots in it.

They say the Emerald Ash Borer has crossed into Iowa, but we don't have them here in the West yet - but we will.

The Emerald Ash Borer will kill all the ash trees in this country - that is too bad. There are so many pretty ones around....it won't be too many years and they will be gone.

I dunno gals. It is weird I tell ya!
Cindy I looked up root girdling and it doesn't sound like that. There were no dead branches and the entire tree did leaf out this spring. The article I found said this:

Certain trees are more prone to this problem than others. Lindens, magnolias, pines, and maples other than the silver maple are susceptible to root girdling. On the other hand, oaks, silver maple, ash, and elm are well known for their ability to form functional root grafts and are rarely adversely affected by girdling roots.

DH and I planted the tree and it was taken out of the plastic pot it came in and roots teased out some, I don't remember how much..I just remember doing it.

This is just such a odd thing and dang we planted 3 trees that same night and this is the one DH had the highest hopes for! The other two are seedless ash and doing great too. Shoot shoot shoot!

I'd take the picture to a nursery or your local county people. We have county people here that know all about stuff like that. I bet they could help you.