Every horse is crooked about something. My horse does it with his shoulders. Some do it with their heads, some with their necks, and others with their hind as your mare does.
It's either a confo thing, a training thing, or an endurance thing.
What I mean by endurance is it takes a lot for a horse to be straight and put together correctly.. so your mare may need extra support and more "micromanaging" until she gets it.
Conformation is just she's built in such a way that it's harder for her to be straight, so again additional support it needed.
Training thing is the rider using too much leg in relation to other aids that the horse yields too far over and therefore ends up crooked.