What led to ex-Philippine president Duterte's arrest?

What led to ex-Philippine president Duterte's arrest?
Former Philippines president Rodrigo Duterte was taken into custody by the International Criminal Court (ICC) on Wednesday 12 March.
The ICC, based in The Hague, believes there are "reasonable grounds" to charge Duterte with murder as a crime against humanity, as an "indirect co-perpetrator" during the anti-drug campaign that rights groups estimate killed tens of thousands.