I don't know if it's the only source, but I've seen those form on live animals in the steel chutes when we're working cattle. A hard impact (probably from head butting in a field too) can cause a light spot in the horn, which scraping or sanding proves to be a delam below the surface. Since a beef or two each year is headed for our own table rather than the market, I've been able to "track" the light spots as I made them into powder horns. Natural horn color makes it harder or easier to spot them.
Kinda hard without having seen a lot of horns to distinguish between a shallow ding near the surface and a deep one started a long time before as the horn was forming. My eye is just not so well schooled. But a clear surface doesn't necessarily mean there's not an invisible deep delam. Recent delams at the surface are easy to get past with scraping/sanding, but those invisible deep ones are stinkers. If you see a light spot forming as you work down into a horn, you can just bet there's a delam down there waiting. Then it's a question of stopping right where you are or working a lot deeper to get past it.