I do it on purpose, whenever I think about it. Like Jill, my mam used to encourage me not to slouch, but my posture went all to pot after I left home and started piling on the weight.
I think I hunched a little in the hope that it would hide my increasing size
(like that works!), and it became a habit that is quite difficult to break. I can't keep sucking in for long periods because my stomach muscles are shot, but I keep practising in the hope that one day they'll return.
Standing tall just feels--and looks--so much better.