her back isn't that long. It seems proportionate in most of the photo's, except the first one posted. If you're sure it's conformational and not behavioural (laziness etc) try getting some more decent photographs of her standing naturally square. Just for a few pictures, it seems she may be sliiiightly sickle hocked. This can all lead to a horse that is finding it difficult to stretch out and ultimately rock back onto it's hind quarters.
The fact that she is also quite downhill in a few photo's, will not help balance either. And she could be purposely restricting her movement from the hind end to counteract not being so front heavy.
If any of that makes sense :)
It's all guessing though, as the pictures aren't that great.