Two factors: firstly it looks as if the resent and current strong winds have got to it giving windburn and, secondly, it is that time of year when the leaves wither and fall off naturally.

Acers have a very shallow root system which extends horizontally along the surface. Those concrete slabs covering the root system could be the problem. Maples are also extremely thirsty trees. Is enough water getting under those concrete slabs to the surface roots?

The pot your Acer is in looks a little on the small side and, ideally should not be sitting dirtily on the concrete slabs but raised up slightly so it can drain. To be honest though you can keep the Japanese Acers in pots they are much happier and easier to look after in the ground.

I think Mg hit the nail on the head. That's the problem.

Thanks BG!