TLDR: it's because the phosphors used to generate a nice broad spectrum white light are falling off the LEDs revealing the base light used to drive the phosphors, which has to be blue or violet because you need a shorter wavelength than the light you're producing, at least for common materials cheap enough to be mass produced.

but scientists have a hypothesis: bright purple light suggests the phosphor layer around the lights has been “delaminated”—peeled off—exposing the blue LED light underneath

This isn't a hypothesis anymore (it likely was when this article was written in 2023), it's been pretty much confirmed: https://www.youtube.com/watch?v=4Mll-YDDAF4

Honestly when this started happening in my city I assumed it was some kind of anti drug use measure.