In its financial results for this quarter, Take Two released a schedule of currently announced games as they relate to its financials. Curiously, the schedule makes a distinction between the digital version of NBA 2K18 on the Switch and the physical version that comes out at an unspecified later date in the fall.
The digital version launches alongside the Xbox One, PS4, PC, PS3, and 360 versions on September 19, 2017. The physical edition for the Switch alone is simply listed as "Fall 2017." All other consoles will be available in both digital and physical versions on the September date.
It is unclear why Take Two is staggering the release like this, though it is likely a problem with Switch cart production. It has been long thought that Switch carts are the reason certain games end up more expensive on the system, so this might be Take Two waiting to see consumer response before committing to physical production.