"The rear-ender is always at fault no matter what" is largely myth. The truth is just that historically it's been very hard to prove you weren't at fault if you hit someone else in the rear. Now, with dashcams becoming more common and people becoming more aware of deliberate-crash scams and road-raging "brake check" incidents, that's starting to change.

It can happen after a sudden lane change where the person that’s rear ended is at fault.

My impression was that was the way most if not all states were set up.

Ummm, that's what I said :)