BigBill said: All I can say is that I think it was unwise to plant the same tree type back into the same hole where the previous diseased tree lived. You will just pass the same disease on to a new tree. There is no way that the disease has been removed, those roots spread out far and wide.
Trees normally live longer then 40 years, unless we are talking about fruit trees or perhaps ornamental cherries, things of that nature.
NMoasis said: You didn't tell us when or how you fertilized, but rapid growth from recent, possibly excessive, fertilization would have been especially susceptible to herbicide damage. If you had a severe heatwave before the tree's roots were fully established, that would also make it more susceptible to damaging external forces.
BigBill said: What type of tree did you plant? Looks like a Silver Maple maybe.
What killed the first tree?
If it were me, I would not have planted anything in the old hole. If it was a disease or another problem leading to its death, that issue is still in the soil. It could kill this tree as well.
It is just a belief of mine that I never plant trees in the summer. I plant in early spring or in mid fall.
Yours could be suffering from too much heat and improper watering. But if that's the case, these problems should clear once it is fully established.