Because Robb Starfall irritated her.
That's the short/silly answer, the long answer is that Selenia had always intended to absorb Alara, but wanted a credible cassus belli to do so. Furthermore, she needed a war and Alara had made itself her primary target after thier interference with her successor, Xerarch Magnus. Sayuki's unexpected defection came just as Selenia came into power at the end of the Xavax Civil Wars. The Xavax were at one another's throats, nobles slaying each other over insults, Xavax was a powder keg that was holding together by a thread and a prayer. The fastest way to hammer the Xavax back together would be to focus them against an external other. She kept Sayuki as part of an extended diplomatic campaign to bait Alara into crossing a line that would justify Xavax declaring war against them, or better yet, the reverse. None of the other rulers were taken in by that, of course, turned out to be a major miscalculation of hers in the end.
I always found that period when the rebellion started interesting, because many of the rebels regarded Sayuki with a sort of "Et tu, Brute?" reaction when he sided with the loyalists.
