danios and tetras can get aggressive if their school size is too small. I've heard 6-7 is the minimum to dilute aggressive tendencies. Chasing in zebra danios is pretty much standard, though.
As for why it's happening now, it could just be a temporary reaction to the change in water chemistry. If it gets problematic, like some fish are getting tired or stressed (pale colors, hiding) let us know. I'm hoping it dies down in a day or so after they get used to it.