I don't consider myself an analyst by any means, but I would imagine a combination of factors kinda converged all at once:
- Addition of Killjoy meant another option for an agent who could lock down a site
- Recent nerfs (happened well before this tournament, obviously, but could be that players underestimated their impact at first)
- As the game progresses and teams have more time to experiment and refine experimental strategies, teams will become more open to trying riskier strategies with riskier agent compositions. When you look at the game, Sage feels like a pretty safe "good" pick and so teams just beginning to practice the game a few months ago probably started with her and built their comps around her. Now, teams have had more time to see if they can make strategies without her work and it looks like they've had success.
I could be totally wrong about all of this, but that's my surface level analysis of the trend.