Because certain tax credits are based on the amount of "earned" income you have therefore as you enter business expenses it will reduce your earned income which reduces your credits which reduces your refund.
The most common reason for this is the Earned Income Credit. (The amount of your earned income can also effect the Child Tax Credit and Additional Child Tax Credit.)
This credit is on a curve that increases from zero earned income to a peak income level and then goes back down and zeroes out at a certain threshold.
Therefore, a reduction in income can cause either an increase or a decrease in your credit amount and your refund.