I think the correct form should be in the present. But, sometimes, some people may not think much before saying that, and they're "trying" to say "What did you say is your name (again)?", but end up mixing "What is your name (again)?" with the past form of the former question. Thus, they say "What was your name (again)?". It's normally a question you ask quite quickly and sudd
"What was your name again?" means, I didn't catch your name when you said it before. Please repeat it. It is a polite expression, with an implied "excuse me."