Perhaps because she & UN helped China identify Uyghur,Tibetan.&HongKong dissidents for imprisonment.

Which is another reason why the US should be very wary of the UN whose allegiances bend toward China in so many ways.

This is more than just an accusation. The UN admitted that the practice of doing so under Pillay was carried out.
Pillay's department at the UN would get requests to attend conferences on Human rights at the UN, and then her department would discretely send the list of dissidents to China.
One case highlighted is that of Dolkun Isa whose family was then imprisoned. His mother died in a prison camp, his father died under mysterious circumstances, another brother got a life sentence but has gone missing, and another brother who got a long prison sentence.

Pillay has had no comment.

Link: https://x.com/Unveiled_ChinaX/status/2108954572660359305