Aside from the possibility of a typo (so recheck everything that you entered), there are a variety of possible reasons.
The first is that the Tax Cut and Jobs Act of 2017 made a number of changes that reduced or eliminated some deductions, like for mortgage interest, state and local property and sales taxes, and moving expenses. Some taxpayers will find that they actually owe more money after the tax reform.
You can find a detailed list of the changes at the IRS website as https://www.irs.gov/pub/irs-pdf/p5307.pdf
Also after the Tax Cut and Jobs Act tax reform passed in 2017, the IRS updated the withholding tables to reflect the tax law changes. However, the tax reductions were not equal across the board - some taxpayers will actually owe more tax - so the IRS urged taxpayers to review their withholding all during 2018.
This means that some taxpayers underwithheld compared to last year.
I don't have enough information on your return, but the tax withholding tables were another thing that changed for your return that may have had a large impact on your refund/tax due.