As the Sunday afternoon of the Austin City Limits Music Fest's first weekend headed into Sunday evening, chart-topping rapper Lizzo took the stage to the screaming delight of tens of thousands.

While festival organizers C3 declined to provide hard numbers for the festival in general or of the crowd for her performance in particular, many guessed that roughly 40,000 fans watched her set in Zilker Park (Lizzo herself shouted out to "60,000 people" on Instagram). ACL Fest's contract with the city caps the festival at 75,000 ticket holders daily and 5,000 credentialed attendees.

Here is the interesting part: Lizzo was not an ACL Fest headliner. It was only 7 p.m. and the "Truth Hurts" hit-maker was on the Miller Lite stage, a slightly smaller stage to the right of the Honda-sponsored headlining stage.

The 7 p.m. Miller Lite stage slot is often booked with a high-draw act that performs right before one of the night's two headline artists performs on the adjacent Honda stage. (The other main event takes the big stage on the other end of the park, this year the American Express stage.)

The size of crowd that Lizzo drew was not at all typical for that time slot, which caused many fans on social media to wonder: If she was such a massive draw, why wasn’t she on one of the big stages?

The answer is a little complicated, and it shines a light on the behind-the-scenes mix of science and guesswork that goes into organizing one of the nation's biggest music festivals.

Let’s start with something longtime pop music critic Stephen Thomas Erlewine said on Facebook: “‪I couldn’t say if Lizzo’s #aclfest set is the best I’ve ever seen at the festival — it’s up there — but it certainly is the most insane. There’s nothing like seeing an artist riding a wave as it crests.‬”

That last part is worth emphasizing.

Lizzo, as the kids say, is having a moment. She was on the rise when her album "Cuz I Love You" dropped April 19; the 2019 ACL Fest lineup was released April 30.

By the time October rolled around, her song "Truth Hurts" had skyrocketed to a record-breaking No. 1 spot on the Billboard charts. As of Monday, it clocked its sixth week in that position. Lizzo's arrival came just too late for her to be booked on a main stage. She even happened to be on "CBS This Morning" the morning before her ACL set.

Lizzo plays again 7 p.m. Sunday on the Miller Lite stage on the final day of this year's two weekends. Another massive crowd is expected.

This sort of thing has happened before in varying degrees since the event started in 2002.

» Photos from ACL Fest: Lizzo City Limits: Were you in the crowd?

In the fest's first three years, ACL Fest had almost prided itself on booking bands that were not necessarily part of that year’s radio landscape.

In 2005, it happened to book Coldplay as a headliner. They were, as Erlewine said of Lizzo, riding the wave. They were big enough to headline. But after getting booked for the fest, they got even more huge, largely via the song “Speed of Sound.”

As C3 Presents principal Charles Attal said in 2011, Coldplay "really wanted to play it; they were great to work with, there were no issues, no egos that we had to deal with. They're all as professional as it gets. They were great, one of our first really, really big bands that we had.” But, again, they were already a headliner — the band just ended the fest as an even bigger draw than it was when first booked.

Other ACL Fest acts have gotten bigger between booking and the actual event. "Royals" singer Lorde became a superstar between the time she was booked in 2014 and the time she took the midsized RetailMeNot stage. "Fancy" rapper Iggy Azalea also had a big year on a smaller stage that year.

But booking a festival is part data reading, part prognostication and part gambling. Big tours are sketched out a year in advance, especially for legacy artists. Acts are often booked six to 10 months before the show. (ACL Fest booker Amy Corbin has said the fest starts booking the previous December and the schedule is set by March.)

A booker must not only gauge the popularity of an artist at the time but also make an educated guess at their popularity half a year in the future, while also trying to gauge the artist’s efficacy as a live act.

» More from ACL Fest: At ACL Fest, Lizzo arrived and Cardi B got there late

While this is happening, a booker is also facing enormous pressure from various artists’ agents, who are looking at the same statistics the booker is (chart position, live draw, fan reaction on social media) and making the best possible argument for the best possible slot for their client.

Which brings us to Lizzo, who drew thousands to a non-headliner stage, and who happens to now have the No. 1 song in America, a song that actually first dropped on 2017 but didn’t start to go viral until this year.

The last time she played Austin was a packed, late-night South by Southwest showcase at Stubb's outdoor amphitheater. And the last time she played ACL Fest? A modestly attended early afternoon time slot in 2016.

Did she have much of a record as a live draw as of, say, this January? In April 2018, for example, Lizzo opened for indie-rock band Haim at Stubb's outdoor stage, with a capacity of about 2,000. The show sold out, but Lizzo was an opener, so it's hard to gauge the rapper's draw accurately.

Let's skip to this year: Her single “Juice” was released Jan. 4; it took eight months to peak at No. 82 on the Billboard Hot 100 chart — not great. But as you now know, “Truth Hurts” is enjoying a healthy reign at No. 1 after 17 weeks on the charts. And since its April release, “Cuz I Love You” has been on the Billboard top 200 for 23 weeks. The album peaked at No. 4 on Sept. 7; it is now at No. 9.

Lizzo's tour was booked before all of this happened.

Another example of Lizzo's new draw: She played the 2,500-capacity Palace Theater in St. Paul, Minn., on that tour with Haim. She returned to the Palace Theater this May for her own sold-out headlining show. On Oct. 9 and 11 this week, she'll go back to that market (her home turf, it's worth mentioning) to play two sold-out shows at the Minneapolis Armory, which holds 8,400. So that’s 16,800 people.

Which is to say that even post-blowup, Lizzo is playing increasingly large venues that she is selling out. But nobody really knew that she was capable of this until, say, June, long after these shows had been booked and stages were locked.

Was Lizzo capable of rocking a headlining stage? Absolutely. Was there any way of knowing, at the time the show was being booked, that would have been the safest possible bet for that headlining stage at that time? Signs point to no.

Could ACL Fest move her to a bigger stage for weekend two? This is a near-impossibility without another act canceling, ACL Fest booker Corbin said Thursday.  Stage and schedule placement is part of the final confirmation of an artist’s contract. “If an artist has significant production (lights, stage settings, projections, etc.),” Corbin said, “that also has to be factored into how the stage is used.”

However, this Sunday, ACL Fest will broadcast Lizzo’s performance on the Honda stage screens after Third Eye Blind’s 6 p.m. set and before Robyn headlines at 8 p.m. (Cardi B played the 8 p.m. Honda slot the first weekend.)

“This will allow people who want to see Lizzo to spread out a little, since they are in the same area of the park,” Corbin said. “We’re also going to improve the lighting out there a little bit.”

Good luck to any and all Lizzo stans on Weekend Two. Stay hydrated!