It's not complicated. PA was always a barometer because they're on the east coast, and polling places normally close there earlier. This time things will shift towards Florida, Georgia and other toss up east coast states that count ballots early.
This is why you hear things like "If PA goes this way, or FL goes that way then the odds of candidate X to win Arizona increase or decrease by this much". That's because if, say, Trump loses Florida and/or Georgia, people in Arizona might decide not to even bother waiting to vote.
Similarly if it's a close contest in the east coast, it might motivate some voters in the west coast toss up states to show up and try to make a difference.