Ask Mr. Smarty Plants

In spite of your feeling that you did no damage underplanting your Quercus buckleyi (Texas red oak), it does seem more than coincidental that the tree began to show signs of distress following the underplantings. This USDA Plant Profile Map shows that while this oak does not grow natively in Denton County, it does in nearby Tarrant, Dallas and Cooke Counties. Follow the plant link above to learn about its favored soils, etc. In particular, we think this extract is important:

"Soil Description: Medium Loam, Clay Loam, Clay, Limestone-based
Conditions Comments: Q. buckleyi is more drought tolerant than the Shumard oak, but less hardy. This tree tolerates alkaline soil as well as neutral and slightly acidic soil."

We have had questions on oaks in your area of Texas recently that we think had to be related to the soil type. Texas has so many soil types and different soils can be just blocks apart sometimes. However, it looks like this oak can tolerate different soils, so we won't blame the problems on that.

On the subject of the insects on the underside of the leaves, this USDA Forest Service Oak Leaf Aphids article has a picture of those aphids, which you can compare with those you are seeing. This should not be a deal breaker for the oak; note that they can be controlled with natural enemies. That is to say, don't spray insecticides because that would kill the ladybugs who are the natural enemy of aphids.

From the Abilene TX Online Reporter News, here is an article by Bruce Keitler that we think hits the nail on the head, Red Oak Leaf Drop Can be Unpredictable. Note that your oak is a red oak.

One final word, when you put all those things under your tree, did you fertilize them? That can be another problem. We don't recommend fertilizing native plants (which your trees are and the other plants are not) because they are already acclimated to the local soils. If you fertilize a plant, and of course your oak roots took up anything you put on those new plants, you are urging it to put on new leaves. This is stress on the plant and in your case, the stress was already caused by the underplantings. It will start dropping leaves to relieve the load.

There is good news and bad news. The good news is that the oak trees will probably survive. The bad news is that the plantings underneath will probably not. Oaks are capable of allelopathy, which is emitting substances that discourage competition beneath them, and will likely cause too much shade for the grass. The ivy and jasmine can take the shade but will try to  spread away from the oak and could become invasive in other parts of your garden. We don't make the rules, Nature does.