Thanks for the notice, Ham. : )
Hi & welcome, Rob. This looks like a combo of the roots having run out of room to grow, accumulated fluoride toxicity, and possibly too much water in a pot that does not have a hole in the bottom.
This is easy to fix. Take it outside in the shade on a warm day, remove the rocks, then pull the plant out of the pot. You'll probably find a root pie at the bottom of the pot. Cut that off by laying the plant sideways and chopping through it with a shovel, or you can use a big knife or pruning shears. Remove the rest of the soil, then replace in the same pot or a new pot with fresh soil so that the trunk emerges from the soil at the same level where it is now.
The partially yellow leaves will probably quickly finish turning yellow, to be expected. After a few weeks some beautiful, fresh new leaves should start to grow from the top. As these trees get taller, the amount of light hitting the top can decrease. Do you think that is a factor? It's hard to gauge from the particular pic.
If the pot doesn't have a hole in the bottom, it would be good to try to find an inner pot that fits just right that has holes in the bottom so you can make sure there's no excess water by taking it out to water, or checking about 30 mins after you water to make sure there's no extra water in there.
It can be worthwhile to get distilled water for this kind of plant because of the known sensitivity to fluoride.
