It sounds like you are talking about the price of bitcoin and the number of bitcoins that are traded on an exchange causing a price drop. Exchanges don't trade their transactions on the blockchain until someone withdraws their bitcoin from the exchange, so you wouldn't see a surge in volume on the blockchain. In short, it is unlikely in the extreme that you would see a surge in volume on the blockchain coinciding with the surge in volume and drop in fiat price.
(It seems like the drop was due to another PBOC meeting with exchanges, see e.g.:
https://bitcointalk.org/index.php?topic=1782476.0)