There can be many reasons, not all are easy to fix like if it actually has a twisted and damaged frame and often cracked. Another common reason is not having the bush opposite the latching mechanism, meaning one side will move back further as it is allowed to, often a reason it both leans and twists.
These are the bushes in the actual hinging area of the frames. These can fall out from damage and allows the seat to become sloppy and exaggerate further issues.
Bucket seat hinge bushes 71_81 FA208F by
Aus78Formula, on Flickr
This is the bush on the inner seat side that the seat back rests on in normal position. If this is missing or crushed it will sit on the latch side and float on the inner side allowing the seat frame to twist until it sits on the anchor bolt itself.
Bucket seat striker bush cable 2 by
Aus78Formula, on Flickr