All root/water related
No comments about the addition of the "salt spikes" that close to the trunk? Looks like no root flares on the left side of the photo.
Doesn't appear to be a "quality" nursery grown item with the hook at the base, large pruning wounds from late lateral cuts at about the 2-3' mark on the trunk.
That size tree should have been a 48" min diameter rootball @ 2500#. Oaks should be dug a little larger than spec.
Grass that lush and leaves that small equal root rot on the tree. Typically we spec a 3-6" positive dig on the install to make sure it's high enough. That lawn looks like a flush or even negative install.
Peel that lawn back without digging in to the surface roots, mulch to retain the moisture for the future, and change the irrigation to reflect longer duration, less frequency on the lawn...you will also have less leaf spot on that turf.