It may be that the banks re-opened after the New Year's holiday week, and money withdrawals became possible again; so that investors who were disappointed with Bitcoin's performance over the holidays immediately started dumping and taking the cash out.
And as we all know, you need to have a bank open to request a withdrawal from a web site.
Here bank transfers are executed only during business hours. If that is the case in Chine too, a withdrawal from the exchange while banks are closed will only lock the money up. Clients presumably will prefer to keep playing until the banks open.