Hi Rajesh,
From your screenshot, I understand that your lowest time-period in your model is Month. Everytime you select Month, Periodid will also be the root for that planning level (automatically). You cannot remove PERIODID as the root (at least, from the front end).
For your problem, it looks, to me, like a bug in the system. It's not because of PERIODID. Please raise this as a message to SAP.
Hope this helps,
Chendur