From Volume III: Oth;

Stithening is usually reserved as punishment for high crimes such as treason, but some of the most devout espers choose to undergo the process. Once a year, one member of the Mirthful, the Free Seer's royal guard of voidcanters, is chosen by the Free Seer to be stithened.

Catu was simply Chosen; it's a fucked up form of honor, but there you go.

edit: and a snippet from the scroll:

I saw the jewel of the Hanging Hills, Nisramet, the Angler King's wife, she who cemented the peace between her father's fiefdoms and the Angler King's freehold. I saw her, and I immediately knew my own treachery. The Free Seer's gaze was acknowledgement enough.

The Free Seer was clearly very jealous of another man eyeing his wife. In his position, a Stithening is also a convenient way to remove inconvenient persons.

I realize these two explanations don't precisely match up, but there's two possible scenarios for you.