She has a set of rules to follow regarding how to sit in chairs. Perhaps they’re a little particular, but most are in the interest of posture, others aesthetic. 

When she’s caught, she’ll insist that she’s sitting up straight. She will tell you how this shows her off better, how it makes prominent the lacing of her corset or the thin fabric over her rear.

And it will all make perfect sense, but so will the added punishment of the top of the back of the chair digging into her stomach as she’s pulled up, bent over it and dealt with properly.