It's a petty reason for treason, but I suppose I would lose my mind too if I had grown men who could be my father bullying me and undermining me while I try to do my job just because I'm a woman.
And if I remember right, isn't it well known that the four lords have shorter lifespans, they can never leave the boundary of the kingdom or the capital really, they have to constantly supply power to the kingdom's shield, they're always a target for kidnapping and assassination etc....are the other nobles really jealous of that life?