It was definitely a lot of seeing through Red Son's bullshit, but also opportune timing. MK had no hope, and Red Son came crashing in with a plan. Mei saw a moment of Red Son's true motivations and saw similarities. And then both decided they were going to drag those good things out of him kicking and screaming if they had to.

"You what?!" Red Son squawked and there is definitely more fwoosh, face snapping towards her, very much bright red at that level of preparation.