Judaism Creation of the Office of Nasi in the Beit Din

[RabbiO]

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.