Well, that's a lot! I didn't expect it to be significant compared to the bitcoin pool.

Do you know how that came to be? i.e how did they convince everyone to start mining namecoin? Or was it just good timing WRT bitcoin?

Merged mining has good incentives since it basically gives free money; for the same work of mining Bitcoin you can mine Bitcoin and Namecoin simultaneously.

I'm not too sure. A lot of people in the cryptocurrency community are saying that something like Namecoin is going to be the second major blockchain-based technology after payments, so maybe it just got the right kind of press at the right time, as you say.