If you've entered an amount from any of the boxes on the slips listed here, you have what the CRA says is self-employment or business income:
T4 slip:box 81, 82 or 83
T4A slip:box 20 or 48
T4A slip:box 28 and you selectedYesto the questionDoes the income declared in Box 028 consist of fees collected for services you provided?in TurboTax Online
T3 slip:box 24 (foreign business income)
Another reason could be, if you imported your last year's return so it got the amount from the previous year's slips.