I thought there was a clear smoking gun reason it has become popular but it doesn't seem entirely clear cut. Except that the blue/teal makes orange (and skin) "pop" as the colors are complementary, and color swapping/enhancing has become much easier in recent years. (And I think, somewhat cynically, because both hues are both pretty far from the default green screen color...)

If you showed me many of the posters shown here in isolation, and asked me to describe the palette, I would have said "that's a yellow poster": https://stephenfollows.com/i/171004131/orange-the-mvp-of-the... - with some exceptions such as Lorax and Unbelievers that I would have said are orange.

EDIT: I changed from "most of the posters" to "many of the posters" as there are some that feel to me decidedly orange.

* How the colors were picked and assigned to each category and (e.g. at what point is red pink and no longer red)

* An indication of distribution in charts, they have different scales on the y-axis.

* The author likely sampled posters with mostly the same color above a given threshold for each category, would that (together with the lack of methodology and error bars) heavily skews the reader's presentation of the data analysis?