So, i have to agree with Bao in that this is just kinda confusing.
So Sinder has been quietly - and successfully - sabotaging people for years now. Nothing really indicates that, had just just kept it up on this level, she would've ever gotten caught.
However, starting, apparently, somewhat recently, Sinder has been seething towards Bao specifically more and more, to the point where Bao has noticed it(described as Sinder being warm and fuzzy towards everyone in the group and getting colder and colder towards Bao, which Bao has been afraid to bring up, thinking she has done something wrong).
This seething makes Sinder go way harder on sabotaging Bao, making Nanoless reach her limit and plan a counter-attack.
And while we don't have any precise timeline for this, for the question Bao keeps asking, "why am i singled out, what did i do differently", i have to wonder...Is the answer just fucking Fansly?