That can happen if you qualify for an income based credit, such as the Earned Income Credit. The Earned Income Credit amount is based on your net income. As your income increases, your EIC (and your refund) goes up, until you reach a certain income level, then the EIC goes down with increasing income.
If you are on the increasing side of the EIC curve, then adding business expenses reduces your income and that reduces your EIC so your refund goes down.
You are required to enter all of your income and expenses. It is illegal to omit legitimate expenses that reduce your income so as to maximize your EIC.
