The biggest source of trouble that led to the deterioration of old coach-built cars such as this AC, was the use of animal glue for the wood frame. This glue fails after contact with water, permitting joints to loosen and plywood to de-laminate. The frame becomes more flexible and this leads to cracking of the body panelling. The panels crack because when the car twists, the unglued joints distort and this over-stresses the panels near the corners of the boot-lid (the welds crack). Water gains free entry to the joints and rot can spread fast. This leads to further loss of strength and stiffness in the body frame, particularly when the boot side panels rot where they bolt to the chassis.
Contrary to popular opinion, a complex car body is not like a jigsaw puzzle. If you dismantle the wooden frame and reassemble it again (with or without repairs), its shape will have altered slightly. Doors won't close easily, while windows and panels might not fit. This is partly due to the amount of movement possible in each joint before gluing. Also, when removing major wooden parts, the remaining frame might sag or twist slightly. This will be worse if the car is slightly twisted to start with. Hand-made wooden parts don't always fit with engineering precision, and everything is finally pulled into shape when the screws are tightened, but springing out of shape when parts are removed.