It may just be a one-off glitch, but it may be a warning sign of problems with the sd-card. When you power off using the correct procedure, the system saves any pending operations to the card. I don't know for sure, but if there's an issue with the card, it may prevent it shutting down.
If it was me, I would backup the data (which I hope you are already doing regularly!), and then do a full format on it.
I would also confirm you are running the latest firmware.