why are the leaves of my japanese maple are withering

Wind burn. The recent strong winds have got to the young leaves. The tree will be OK, though. The leaves on many of our red leafed acers are also withered. If this has not happened in the previous eight years then you ahve been very lucky.

I agree and I think I would run a hose beside it for a good while to give
its roots a really good drink and also add some blood fish and bone as a feed just to help it on its way to recovery

A couple of mine have leaves that have withered due to lack of water. I thought they had been getting enough moisture as there had been showers, but it was not penetrating deep down enough for the roots to get the benefit. Where I am we have had a very dry spring indeed. I am hoping they will recover.

As mentioned by Bulbaholic and Inverglen, it can be caused by strong winds as well, and I have had this happen too.