At around 07.11.2020, 16:30 UTC (one and half hours ago), it seems like many news outlets in the US decided to call the 2020 presidential election for Biden, which prompted celebration, congratulations, and so on. This seems to root on the Associated Press and Edison Research deciding that a win for Biden now surpassed some magical threshold of certainty, in particular due to the projection that Biden won Pennsylvania (which would give him sufficient votes in the electoral college). So far, I get this.

But what new data caused this change? Again, going by many major news outlets, the situation in Pennsylvania looks pretty much the same as 24 hours earlier, with about 5% of the votes left to be counted and Biden having a lead of 0.5 percentage points or about 30.000 votes on Trump. None of the reports I have seen so far mention data about another bunch of votes coming in or similar. So this seems to come pretty much out of the blue (at least on a short time scale).

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    There's this on the AP's website, but it doesn't specifically say what set of votes triggered the call: apnews.com/article/…
    – IMSoP
    Nov 7 '20 at 18:17
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    It is so very important to remember that projections made any news outlets are not official. Despite AP projections, votes are still being counted in Pennsylvania, in Arizona, and even in several states that the AP declared decided at the instant polls closed. I am not disputing the AP's projections; the AP has been very good for over 100 years. But please remember that those projections Are. Not. Official. Nov 7 '20 at 18:41

On the FiveThirtyEight live blog, Nate Silver had a discussion at Nov 7, 9:39am EST ("So, When Will We Get A Projected Winner?") about when networks will project a winner. He quote that the Associated Press will not call a race if the margin is within the mandatory recount range for that state, and that for Pennsylvania, the recount threshold is 0.5%. Indeed, many networks called the Pennsylvania race (and thus the election) at around when Biden's lead in Pennsylvania hit 0.5%.

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    @Acccumulation, Alaska still has approximately 45% of its ballots uncounted (for whatever reason, they've decided to delay counting mail-in ballots until November 10). When Pennsylvania was +0.5 for Trump, there were still many ballots uncounted, most of them in Democrat-heavy areas.
    – Mark
    Nov 8 '20 at 1:11
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    @Acccumulation what makes you think there is a “standard”? The networks call states when they feel they wont have to backtrack on that calling - to backtrack a calling means viewers lose confidence in that network, so the networks only call when they are very certain. They each have their own standard for doing so - Fox called AZ much earlier than CNN for example.
    – user16741
    Nov 8 '20 at 2:52
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    @Acccumulation From Silver's quote from AP: "AP will not call a race if the margin is within such a mandatory recount range — or if it could fall into that range as final votes are counted." (emphasis mine) The difference between Alaska and PA is that AP and other's think they know enough about the remaining votes in PA that they can confidently say that it won't swing back into the recount range.
    – jkej
    Nov 8 '20 at 12:19
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    How can they know that? They know what counties the remaining votes were cast in and what type they are (absentee, provisional, overseas, etc.) and they can compare that to previously counted votes with similar characteristics and estimate how many of those votes will go to each candidate. Such estimates are much more robust in PA than in Alaska because a much larger percentage of the votes have been counted.
    – jkej
    Nov 8 '20 at 12:30
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    @Acccumulation The question asks "what changed", and that is what changed. The networks went from being confident Biden would win, but not able to call it due to this rule, to being confident Biden would win and able to call it. It would not be a sufficient condition on its own, but since all the other conditions were already met, it became one. Nov 9 '20 at 8:54

As FiveThirtyEight pointed out in their livestream, that was the point where Biden's lead in Pennsylvania went over exactly 0.5% (rather than being rounded up to 0.5%), with the trend still moving in Biden's favor. Specifically, Allegheny and Armstrong counties reported votes, with the Allegheny votes being 80% in favor of Biden.

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    The important thing about 0.5% is that’s the recount threshold. Below that, there will be a mandatory recount and the AP won’t call a race if that’s the case
    – divibisan
    Nov 7 '20 at 18:16

Data from Pennsylvania had been trickling down all day. CNN for example had been commenting these minute changes hour-by-hour (arguably hyping them a little too much to fill the time). Shortly before calling the election, they reported an additional increase in the gap between Trump and Biden in Pennsylvania, something on the order of a few thousands votes. No matter what kind of model you are using to make a “call”, if vote counts are reported in small batches, the situation after crossing the threshold will necessarily be pretty much (but not exactly) the same as it was before.

Once that last batch of a few thousand vote was released, networks make their own decisions (I believe CNN might have been first, Fox News took 15 minutes longer) based not only on the raw number of votes but also the type of ballots that remain outstanding, where they come from, possibly local election law or the proportion of votes going to one candidate among other similar ballots. Some sources (include the AP) provide some insight into the kind of things they take into account but nothing specific enough to reproduce the model or determine the threshold in advance. Since a Pennsylvania call would have been dispositive, they would also take a few minutes to get senior people to sign off on it.

It matters a lot less than the headline result and Pennsylvania made that less relevant but note that as of writing, Biden has been projected to win Arizona by some sources (AP, Fox News) but not others (ABC News, CNN). Unlike Pennsylvania or Georgia data, I think the results from Arizona are released once a day.

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