Looks like root problems, if the clay hole was back filled with a sandy loam it is likely the tree drowned, clay bottom acts like a big un drained clay pot, I would recomend replanting with a much younger tree so it can adapt to the clay as it grows, they have done studies and a 1" tree will usually be twice the size and health of a 4-5" tree at the 7 year mark the reason being is it takes 3 years for every inch calaper for the roots to adapt and establish, and a more larger tree has less adabtability than a younger tree.
also only dig the hole as deep as the root ball is this prevents the settling and in clay the comode bowl effect, also consider planting shallower in clay and 3-5 times the root ball size hole, but back fill with existing soil. and no deeper than the root ball if anything slightly shallower.
Oh and the drought could have also caused the drowned roots if the leaves are scorched they are not feeding so there is no transperational pull so no where the new found water can go, a drought stressed tree needs deep infrequent waterings so it can take it up over a week or so till the leaves open the stomata and start feeding again. In other words in nature how often does it rain enough to fill the hole not every day, or even every other day, so try to pattern your watering to what the tree would get in in its natural enviroment. (normal rain fall average)
Paul