Because they’re obviously ex-girlfriends with years of unresolved angst, Mira utilizes the colony takeover as an opportunity to visit Wash at Boylan’s. They deal with their past in a variety of ways and Mira thinks maybe, just maybe, this’ll serve as closure before she goes back to 2149.
But then Mira happens to witness Wash die. Except, Wash isn’t actually dead. She’s too valuable to just kill off like that. Even though the writers may not think so. It was a stunt staged by Lucas to throw off Taylor’s rationality. Wash is actually alive, but a blank shot still does damage. That’s why they stash her in the infirmary until they can find another use for her. Turns out Mira gets there first.