Need a bit more info please - did you plant it in the ground, and if so, what preparation did you make beforehand, as in did you dig the area over thoroughly and add anything to the soil before actually planting? How often has it been watered since you planted, or has your weather been very wet during August and September?

In the middle pic there seem to be some twigs with dark clubby (club-like?) lumps on the end? Is that what they are?
One of the worst case scenarios is fireblight. Check for any oozing from the bark. It can hit quickly so I pointed to it as a possibility so you might be able to get in early on it.
Another symptom is hook shaped twig ends.
Good luck!

It's in the ground, we have clay soil, we dug a hole but didn't add anything to the soil. We looked up Crab Apple trees before we bought it and they seemed to be easy to grow in any soil/area etc. We've watered it since planting but not overly, we were away for a week, we've had quite a bit of rain, not a lot of hot sun. Our other shrubs in the ground, mainly dogwood, cotoneaster, pyracantha, are all doing okay with no problems.

No apparent reason but the only thing that comes to mind is the planting hole in clay soil. The hole needs to be big and the soil in the bottom of the hole should be loosened and grit added to maintain drainage. Otherwise the hole will fill up with water, depriving the roots of oxygen.

Was it bare root or in a pot? I'm guessing pot as it's fruiting so well in the 1st shot. Did you buy it in fruit?
Other trees in the area that might have similar problems, if it's an airborne infection, would include apples, pears & hawthorn.

Is it all dying sunny? Or just some of the branches. I’m sad to see this. It was cleary a really beautiful tree. I do hope it isn’t a case of Fireblight. New trees do require a great deal of watering for the first year...hopefully it is drought and the tree will recover. The fruit are like jewels!

P.s. have you checked low on the trunk for damage that might have been caused by a hoe etc?

Make sure any mulch isn't in contact with the trunk too.
From all the things you've said I think it might need to be dug up as the pot compost is draining but the hole is forming a cup & the roots are not getting enough oxygen & too much water causing rotting. Be good to have a close look at the roots too

It was in a pot and in fruit when we bought it. It has new growth. We're thinking digging it up may be the answer to see how the roots look - would it grow in a really huge pot???? We get a lot of rain around here.

One of the problems with trees planted from pots is that if the roots are in any way constricted (and they almost always are) by the pot, it is very difficult for them to grow away into the surrounding soil when planted out. Also if the soil is not as fertile as the original compost, the roots do not always grow away from it. Consequently despite good watering the tree suffers from drought and starvation. We have had this with a few thing we planted. If the tree has only been in for a short time, then the answer may be to carefully dig it up, tease out the roots (basically almost bare root it) and replant as suggested by Jimmy.
Good luck with it.

I suspect the problem is the clay soil and most likely insufficient water. If you're going to plant a tree in heavy soil, the best thing to do is dig over an area much larger than you need to plant into, preferably adding plenty of composted manure or good garden compost at the same time. Let it settle for a day or three, then dig your hole and plant the tree, having first teased out the roots. I say teased, I once saw Alan Titchmarsh demonstrate that using a tool like a hand held rake with 5 sharp points, which he literally ripped downwards from top to bottom most of the way round on a particularly coiled root specimen, but either way, the rootball needs a bit of disturbance before planting.

Unless you've had lots and lots of rain, it might well be suffering a bit of drought - a rootball extends a good six inches or more below the top of the soil, and it takes a lot of rain to keep that moist - in the meantime, your tree will still have pot shaped roots and is unable to seek out its own water. The fact that fruit was present also means a higher requirement for water.And your other, older large shrubs are fine because they have extended their roots all over the place to seek out water - your new tree has not had time to do that yet.

As its been in such a short time, I agree with Owdboggy - should be possible to remove it (carefully) dig over the area as described, tamp the soil down (instead of waiting for it to settle), then dig the hole and replant, at the same level as it was in its pot (not deeper) having teased out the rootball. If the rootball is dry, sit it in a bucket of water while you do all this, if its not dry, just keep the rootball covered while you sort the area out. And then, water thoroughly and keep watered up till the weather becomes properly autumnal, chilly and damp. It'll need easily a couple of gallons every week, unless you're having regular torrents of rain (not just a few showers) and again in spring summer next year if you have any dry, sunny and especially windy and dry spells.

Apologies Jimmy, Scrolled too fast! I just repeated what you said, d'oh!

Thank you to everyone for all of this help and advice. We've carefully dug the tree back up. The trunk is totally fine. The rootball was quite dry but the roots were showing nicely and trying to grow outwards. The soil was dry. We put the rootball in water then put it back in and we've given it two buckets (12 litres) of water. I plan to water it every day for a week or so, then weekly, which I now realise I should have been doing all this time.
We have had rain recently but we've also had wind, our garden has always been soggy, a few years ago we had to pump it out as it was so wet, but the weather has been so strange this year.
Hopefully, the slogan, "Life, Just Add Water", will work, fingers and toes crossed!
Thanks again!

Hurray! I do hope it recovers Sunny. All of this thread has made me realise that I probably killed a new apple I bought a couple of years ago by planting it from a pot in to clay...so it has been very useful for me too!

Thank you for making me feel better Cottagekarer, gardening is a funny old thing, you can feel so pleased with yourself somedays and so darn stoopid others, just like life I guess!
As soon as it looks healthy again I will post photos!

I certainly identify with that Sunny! :)