The high court wasn't open many times, usually, when there was a dispute among the nobles, they would be sent to a normal court where it would be held by a priest. However, a high court takes things further by handling the most important case.
Requiring the pope himself to judge the truth from people. On this day the high court had open for a special occasion. To judge a prince on his crime of colluding with a heretic, the main people that accuse him were the Antores household.
A month ago, Brent had mobilized his force and exerted some power as a prince to pressure the Antores family. Using his authority to remove Reeva from his position as the last son of the family. Of course, the prince and his influence alone couldn't do it. The other family had a hand in this too.