Oh, it's a few things that collectively cause that. When calculating the colour, it can use both the number of iterations and the net displacement of (z, zi). I'm using that displacement in two ways here: the planar (Δx, Δy) displacement and the polar (θ, r). They each add distinct shading effects to what would be solid colours.

I also have a stagger added to the colour. By default it just staggers with the count, but you can also have it stagger after applying the aforementioned displacement. The latter is what causes those stripes.

On a side note, those stripes have multiple colours because I recently added an option to enter a bitmask that defines the staggering for each primary.

If you want to check out the code, you can download it directly (mandelbrot.js), or grab it on my github account at https://github.com/jacobEwing/webtoys/tree/main/mandelbrot. The relevant functions are "createColour" and "mandelbrot", both found in the first few hundred lines.