Without looking at your return it is impossible to know why.
These are the most common things that effect your refund or balance due:
- Change in Filing Status;
- Change in dependents claimed;
- No longer have a child, under age 17, to claim the Child Tax Credit
- The amount of federal and/or state withholding;
- Income based credits, such as the Earned Income credit;
- Tax bracket changes
Here is a TurboTax Help Article that will give you more information..