Something about being made of 'soot' is held together by having a strong sense of self/willpower. We've already met a 'person' who seemed to be made of built up spare soot, who had a perfectly strong personality. Shirley being so quiet was a sign she was in weak and failing health all along; she had no 'herself' properly built up, and now there's just....
....not enough left to hold together at all. Heck we even saw she could easily have triggered her own 'cage' to drop almost immediately, with almost no effort. And we know it's easy to leak soot if you're upset, which....
sort of makes it seem like she felt *nothing*.
She was empty. And maybe not even being able to put forward enough energy to free herself was the final straw?
Edit: I went back to re-read, and I think on the page where Kate does her angel wing trick, we see Shirley reaching out for Ram's hand.... so that might be the very moment before she starts to dissolve?
Was it something to do with seeing them arrive....?