Same things happened to me before for the name RUV.com. I won the name. they charged my CC. After 1 week, when I looked at my portfolio, the name was gone. I contacted NW and they said the name was actually a transfer not a drop. Just after 1 week the name went to Register.com.
The reason was probably ex-owner somewhow managed to renew the name even though he could not and Register.com and the ex-owner forced NW to back.
My advice is Lock the name, even though NW can still can take the domain if they forced.
Another but alittle risky option is Transfer the domain from Dotster to another registrar before they can take it out of your hand.
SAF