it's the time of year for tree leaves to start changing colour before the fall but some ornamental acers can suffer wind & sun burn. Would need a photo if possible to see with more certainty what could be the cause.

Bet it's got dry. Wind and sun together cause the leaves to dry out faster than the rootscan supply water, so the plant cannot support the leaves. It'll probably be fine next year, but find a sheltered position, not necessarily out of the sun, but away from cold and drying winds. Phil J

We've had a howling gale for the past 48 hours. I fully expect our acer leaves to look desiccated and drop off! It is early for normal leaf fall but this has been an odd summer with drought and then heavy rain - I've noticed a lot of trees are shedding their leaves already.

agreed Moon Grower, got a little worried last week when my two 15' Norway Maple's leaves started to turn brown, too early as you say but as Philjeffs says they should be ok for next year.