The chapter is part of a larger "live service journey" for the game that begins in earnest in December. The first phase ("Overture," which lasts until January) brings a free-form Practice Range mode, a single-player story ("The Last Tiger"), vehicle customization and tank fights in Belgium. A second "Lightning Strikes" phase (January to March) will bring a co-op Combined Arms operation, the return of Rush competitive play and a Squad Conquest mode. In addition to battle royale, the spring release will add the Greece theater, a Combined Arms mission and an unnamed multiplayer mode. There's more coming after the spring, as well.
It's not clear what prompted moving battle royale to spring, but it might not be a bad thing. The genre is increasingly crowded between titles like Fortnite, PUBG and Call of Duty: Black Ops 4, and BFV risked being overshadowed. There's still a risk of that in spring, but it might offer some breathing room. The extra time should also provide more opportunity for polish that ensures a smooth launch.