The reasons changed many times over the past two weeks. First it was a pretender to the throne, then a demand for democracy/republic government, then complaints about oppression of freedom of speech/opinion, then complaints about the conduct of the war, "negotiations with Tara", the friendship with Coria, etc.
The Duke's son Zak (in infiltrator) was recently executed by CE. It kinda set him over the edge.
Essentially it comes down to a desire power, like everything in BM
The "Rieleston" name is brilliant, however. They even chose Eston's old banner.