Although EA has already had its hand on Battlefield websites two through to nine, It seems they've decided to 'preserve' their future a little longer and purchase 8 new domains, starting from www.battlefield13.com to www.battlefield20.com.
EA has strangely missed out on Battlefield 10-12, but I'm certain most of the discussion will be about why EA has secured such websites.
It's only been very recent that EA purchased Battlefield4.com for $9 grand, so for them to buy out all these domains now is very strange.
Perhaps EA decided to go on a spending spree, just to be safe and ensure they don't have to fork out another $80k on more websites just like they did for Battlefield.com.
Or maybe these numbers mean something else. If you count every game since Battlefield 1942 to Battlefield 4, that makes a total of 12 games. Making the 13th (possibly Battlefield 5) title have relation to the domain www.battlefield13.com
The second theory is that EA have decided to a Madden and bring out a new game in the franchise every single year and changing the title names to Battlefield 13, Battlefield 14, and so on. The second theory makes EA sound like they might just be turning their favorite top selling FPS into a seasonal sports championship though, so perhaps they'll avoid that route.