This has puzzled me since I first noticed it as a teen. I have significant stretchmarking on my lower back and hips. On my back, they are broad, horizontal stripes. On my hips, they’re both horizontal and angled, but taper much more sharply.
I’m a modest 5’10" and 134lbs, and I’ve never been overweight. If anything, I’ve been significantly underweight for most of my life. So what would cause these curious marks?