This came from out of the blue but I heard some chatter of the instability of the upcoming league so a major shift. However it is business as usual since the team plans to unveil their kits Friday. We will be in San Francisco for the City match instead.
The Beautiful Blog can say more. The plan is to still be there for the friendlies.
https://blog.sfgate.com/soccer/2019/06/27/oakland-roots-will-join-nisa-drop-out-of-npsl-founders-cup/
Oakland Roots Statement: League and Membership (oaklandrootssc.com)