@Ohgodpleaseno : Basically
Dia's family was on the side of royal family that lost, and so paid a lot in reparation to the winning side.
But then the winning nobles pushed for more, specifically for Dia herself in order to get children with strong magic power in their blood line.
Initially Dia's family was going to accept, but the retainers didn't and basically 'revolt' to protect Dia and her father, with the civilians joining in once they know of the situation.
But knowing that the nobles will crush his retainers and citizens if they clash, Dia's father order their own assassination to break the people's morale.
tl;dr The knights refused to let their lady be sacrificed to the greedy nobles, civilians agreed, and Dia's dad decide the only way to stop them is heroic sacrifice.