Musk was definitely strategic with the announcement. He was speaking in front of the National Governors Association, which is full of politicians looking for economic opportunities -- he's no doubt hoping that governors will jockey for a Gigafactory (and offer incentives) in their state. At the same time, though, it may be a realistic forecast. Musk expects most new cars in the US to be EVs within 10 years, and to virtually dominate the market in 20. If Tesla doesn't have enough factories in place, it risks losing business as electric transportation hits the mainstream.
The talk also saw Musk weigh in on a few other topics. He's not opposed to self-driving car regulations, but he believes they should expire as technology evolves. Also, he believes that car security is a high priority. He notes that Tesla cars already have "special encryption" that protects vital systems like the powertrain and brakes, and he's entertaining the idea of a kill switch (of sorts) that no hacker could touch. Don't count on governors heeding his regulatory advice, but it won't be surprising if future Teslas are better-equipped to deal with online threats.