I had a customer in the store yesterday with the same issue. Two things: 1, I would suggest a software update. 2, are you sure the NOOK is shutting down completely and not simply going to sleep? You could adjust that by going into "Settings" and making the sleep timer last longer; 2 mins, 5 mins, 15 mins, 45 mins, an hour.
Other than those suggestions did anyone else have this problem? The person who had this issue was on their 2nd NOOK device... it's one thing if an initial device has errors (shame on me), but two of them?? Memory was mostly free, full battery charge, etc.
Are you sure its shutting down? If you don't click for a while it goes black then if you hit the n at the bottom it opens again.