This happens because you may have some self-employment income as part of your return. If you have a T4A as part of your return, this is likely the case.
The Canada Revenue Agency (CRA) considers amounts in box 20 and box 48 of the T4A slip as self-employment/business income.
If you entered a T4A slip in your return with amounts in box 20 or box 48, then you must complete Form T2125: Statement of Business Activities.