They get by with their raw intellect for most of school, and everyone compliments them on how smart they are. But they never learn skills like how to study, how to learn, how to persevere etc.
So once they hit their plateau, which everyone does, they don't know how to get past it and then they give up. A kindergartener that can read like a grade 8 will work off her raw intellect until she gets to grade 8, but then once her abilities max out, she won't have learned how to get past that, and that's where she flames out.
It happens all the time, even to friends of mine in high school. One of my gifted friends did well in high school but unfortunately flamed out in spectacular fashion in freshman year and I haven't heard from her in 30 years, even though we were extremely close and I reached out to her several times over the years.