When a dependent has unearned income (income not earned from working, such as bank interest, prizes, hobby income, and so on) that is taxed at a higher rate. It is often called the "kiddie tax" and is due to a tax provision that is intended to prevent parents from dodging taxes by putting savings in their child's name. While that is how it originally started, it applies to any unearned income of a dependent.
https://www.irs.gov/taxtopics/tc553
