Things like - lack of water, located in a windy icy place, scorching sun, frost pocket can cause this to happen.
Thank you for your responce.However non of the above seemed to be the issue. When I put my glasses on & had a good close look,I found a number of scale type things on the branches.I isolated the prone trees, washed their root branches & pots. I then sprayed the trees with a prescribed insectacide & repotted in new soil. I hope this works as I love my Acers.I have about 9 but only 3 were afected. Thank goodness.
Scale insects; if only a few, wouldn't have such a dramatic effect.
Another thought...hows the root ball, you had it out the pot. Just inspected a poorly acer, which has been very slow this year and I 've found a vine weevil grub has eaten the roots.
However, you've done the right thing and washed them off.