It's an artifact of how Rough.js renders sketchy lines. It does that by creating some random offsets. For some random values, it may create this effect for long lines, which is not ideal. This is less pronounced in shorter lines. The latest release has tried to reduce this effect, but there is a small probability that it may still happen: https://roughjs.com/posts/release-4.0/

It's how rough.js approximates hand-drawn sketchy lines. It looks great in some cases, okay in many others, but if they diverge too much, it looks weird. I think the intent is to evoke the style of sketching where everything is stroked multiple times with short lines.