I suspect Tanigawa knows something (that the ghost version of her may have forgotten) that she believes would put Yamano in harm's way if he were to pry into Hori's case, and she's playing the "villain" to protect him.
You know what would be fun though? If she also time-leapt after the Sasakura murder incident, but thinks she's the only one who did. I myself find that rather unlikely though