Judaism Creation of the Office of Nasi in the Beit Din

[Peter]

In 191 BC, the Beit Din created the office of the Nasi following a nonconfidence vote.  What was the reason for the nonconfidence vote which removed the Kohen Gadol from the Beit Din?  I cannot find any information as to the reason for the nonconfidence vote.

I would appreciate any information in this regard.

 Onias III.

Yes, I am aware that Onias III was the kiohen gadol who was removed from the Beit Din Gadol, but do you know why he was removed?  This is what I am missing, is the reasoning as to why Onias III was removed.  His father, Shimon II was the last Kohen Gadol to preside over the Knesset haGadolah and when the Sassanid Empire took over they abolished the Knesset haGadolah and Onias as ex officio presided over the Beit Din Gadol but then the Beit Din Gadol vote him off by a nonconfidence vote.  What was the reasoning for such a vote in 191 BC?