China central bank had reduced offshore CNY supply recently.

The overnight rate for offshore CNY on 20 Sep 2018 was 2.953% but it jumped to 4.562% after China intervened in the money market using its SLF & MLF.

China central bank had created a shortage of CNY offshore and caused the overnight rate to spike tremendously.  This is to prevent the renminbi from depreciating further because investors will find it more expensive to short the CNY.