A picture would be handy!

It could be any number of things e.g.

If it is a grafted specimen the the leaves could be coming from the root stock rather than the grafted variety.

If the spots are indeed orange rather than red then it could be 'Coral Spot' which is a serious disease that affects shrubs and trees and may have killed it!

I hope I am wrong for your sake but it might be worth googling for an image of Coral Spot and compare it with the symptoms your tree has....Tg

http://www.thegardenersalmanac.co.uk/Data/Coral%20spot/Coral%20Spot.htm

Without a photo I would agree with TG the leaves are coming from the rootstock not the scion

If the growth is low, but still on the main trunk, remove the part of the trunk above the growth immediately. It has got dieback, and may now have coral spot, so cut it off asap. With any luck, the rest of the plant may recover. If your Acer is in a pot, it would be wise to check that its not rootbound, if you've not repotted for a couple of years.

Now that I see it I am not so sure it is 'Coral Spot'

The spots look too small and it looks like the natural markings that you get on the bark of some trees.

Plus the fact it looks as if these markings are on the 'rootstock' and not the scion.

I have just been out to look at my acer rootstock, and yes it has similar intermittent dashes rather than dots on the rootstock, and not the scion!

Plus I have noticed that I have fresh green growth coming from the rootstock (must get out and cut that off soon)

Looking at the picture again, I get the impression that the shrub is in a pot, so it may have suffered a physiological problem, possibly brought about by any of a number of things e.g. drying out, getting saturated, getting overfed, wind damaged or a permutation of the lot!

I'm sorry I just don't know what the problem is exactly.

BTW I have 'googled' for some coral spot images and found these and I think you will agree that they don't look like the spots/markings you have.

http://www.first-nature.com/fungi/nectria-cinnabarina.php

Teegee's right, its not Coral Spot. This is not abnormal on Acers, this kind of bumpiness on the trunk, particularly on older wood. But Acers are prone to getting phytophthera, causing dieback, so you should still remove the part that has died back, back to healthy wood.