I don't know if it is normal, but my Nook does this as well.
Some of the Nook cases have a magnetic closure that also "wakes and sleeps" the Nook. I know the Poetic cases are one of these as I had one. Whenever I moved the Nook when it was in the case, it would wake up. It caused the battery to drain. I quit using the case and no longer have the problem. Luckily I recognized the problem soon after purchase, so I was able to return the case.
I have seen others have this same problem with other cases with the "auto wake" feature. Try taking the Nook out of the case for a day or so to see if this is the issue.