Has it been very hot and dry with you? If so it could well be lack of water alternatively it is just dying back naturally. Acers lose their leaves in autumn.

It does seen a little early, have you had strong winds? I believe they prefer a sheltered spot, not too much sun or wind.

Yes Heron they need semi-shade, no sun on them in the summer months from 11am - 2pm and definitely do not like wind at all, though if they are kept well watered can cope with. I'm amazed that ours aren't looking shrivelled from the strong winds we have had and can only assume it is because the soil is so wet.