I like doing mindless things sometimes. It's calming, makes me feel more stable. The squiggles are one of those things. Some variations of simple algebra also work and writing out permutations of strings of numbers or letters (yes, I know that doesn't seem mindless, but it is for me). I once wrote out all the different ways to rearrange the letters in "aardvark" but I lost the notebook so I can't prove it anymore. It took me a large chunk of senior year in high school.
Anyway, the squiggles were happening long before that. I don't make the squiggles, they happen when I let my mind go blank with a writing utensil in my hand. I do have to deliberately start them, but they make themselves without conscious guidance. I keep the ones that look interesting in the end. I do them mostly on post-it notes now. I used to use pages of my sketchbooks, but I would always start thinking before I finished a page. I didn't like that, so I went to a smaller size.
I mostly use sharpies now, I used to use pencil but I hated having to stop sharpen them or even to click the mechanical ones. The squiggles are all single lines that tend to cover the entire post-it. I don't keep many that don't cover it entirely, I don't like when they feel incomplete.
They tend to reflect my physical rather than emotional state I think. the second one down was done when I had a stomach ache, for example, the first one was a headache.