Maybe the original soil ball dried out even though the soil outside of it is moist. Poke around and have a look. Summer is not over, tapering off the water may have been premature. Water enough to keep all the soil the tree is growing in moist under your conditions, not on an arbitrary schedule that may not pertain to your soil, climate or weather at all.

Could you have overwatered maybe? I planted a new Autumn Blaze Maple this summer too. I check the soil often and it rarely needs water. Still looks great though. I have some Whitespire birches though that could use a good drink about every 4 days. Keep checking your soil. If the leaves already fell off, I'm guessing you're done for this year, but that doesn't mean your tree isn't still storing energy.