TurboTax (TT) uses part of your tuition expenses to claim a tuition credit. Since you are not allowed to double dip and claim a 529 plan tax exclusion for the same expenses. some of the distribution is taxable.
For a full explanation see this link below; which includes an example.
https://ttlc.intuit.com/questions/2662573-where-do-i-enter-1099-q?jump_to=answer_4402991
