The Wrestling Observer Newsletter notes that the friendship angle between Rousey and Natalya has been dropped for weeks but the plan was always for Natalya to turn on Rousey, around this point in time.
The heel turn was dropped when WWE Legend Jim "The Anvil" Neidhart passed away back on August 13. The feeling was that they couldn't turn Natalya anymore once her dad passed, which is why they went back to Nia Jax for the feud with Rousey.
Rousey is scheduled to defend her title against Jax at WWE TLC on December 16. Rousey already defeated Jax at the WWE Money In the Bank pay-per-view.