Are you saying that you added 1.5 feet of soil over the roots? That could definitely cause problems. How big around is the retaining wall?

Alex

Yes, we added 1.5 feet of soil over the original ground level. The retaining wall runs parallel to the fence against which this tree is planted.

Why would adding more soil around the roots cause this kind of problem?

you should never add dense soil over the base of the tree, they have to breath. You can add mulch. If you need to add dirt start a few inches from the base.

persimmonbob is correct. The feeder roots of trees are in the top few inches of soil, which has much better aeration than soil 1.5 feet down. Another potential problem (that takes longer to manifest) is that the bottom 1.5 feet of trunk are now below ground. Some trees respond to this by growing roots out of the trunk, other trees can't make that transition, and their trunk will eventually rot. I don't know how your tree will respond. The best thing for the tree would be to remove the extra soil. There is a good chance that the tree has begun sending new feeder roots upward, so if you do remove the soil, keep an eye out for roots to avoid damaging them.

If you don't want to remove the soil, I would seriously consider just planting a new tree this fall or spring. The old one may never fully recover, and even if it lives, it may get a rotten trunk underground in a few years.

Alex