daily trading volume of bitcoin is nearly $10 billion now and back in September it was even more
same with bitcoin cash it is currently at $500 million.
i don't think such dumps can cause such crashes specially since people selling this much size are either selling it off the exchanges because of tax problems and lots of other concerns or they do it little by little to both evade those problems and also not crash the market.
not to mention that by late September price was $4000 and rising towards $8k then $10k and finally $20k! how could he be dumping and crashing the market while price has been rising?!