This may be a problem with how you turn the steering wheel.
Most likely you are currently pulling the steering wheel which is pulling your torso away from the back of the seat. You are using your core to hold yourself in position.
You need to push on the steering wheel to turn. The hand/ arm motion is a mix between a pushup and a cross (punch). You should be able to feel the steering wheel flex under your hand.
If done correctly you don’t have to wrap your fingers around the steering wheel. Just use your palms.
This will force your upper body back into the seat and you will stay fixed in position.