Without access to your tax return we can only speculate.
By far most common reason you would have received a refund, is that you had a tax payment you didn't report on your tax return. You either made an estimated payment or applied your prior refund to your 2018 tax returns.
It is also possible one of your income sources directly reported taxes withheld that did not get reported on your tax return.
I advise you to review your source documentation to see if you can match to a tax payment reported on one of the forms.
