One objective thing that happening in September is tax loss harvesting, which basically create a huge incentive for investor seel they stocks and investiment in loss to have tax-benefits in the end of the year. So in September, almost all large/medium investors will liquidate some position. So if you have a big incentive to liquidate stocks that are no longer doing well, the whole market will suffer.
I think that is one of the main reasons for this monthly drop every year.
Interest rate adjustment is one of the Cardinal propeller of the asset market in it entirety because Bitcoin falls between that assets that rate adjustment greatly influenced the Price, although at a point the market available liquidity still make up for the great number of how the rate change impact on the market and if you see the month of September have been filled with a lot of market instability for Bitcoin and other Major assets, but still at that, Bitcoin still manage to sustain it market value and at this point Bitcoin have made several attempts to initiate a bull market run despite all the crisis in the forex market.
This is why we can be confident that Bitcoin is going to make some positive market movement pretty soon and if care is not taken, we may likely see a 70k Bitcoin Price before the end of the month of September.