I second the idea of going line by line. I have an accountant, and she always sends a last year/this year comparison as part of the packet of stuff. Could you have lost a dependent or a deduction along the way?
This year, we got slammed on the kids' trusts. We control them until the kids turn 30 (youngest is 19, so for a while). Normally, they're just "there", but last year, we switched financial advisors. The old guy had the kids in stuff that couldn't be transferred in kind--normally, if you hold, say, Apple stock, and transfer it, you still have Apple stock and no capital gains are incurred. But, we couldn't do that, and the kids incurred capital gains to the tune of $4000+ each for the younger three (less for my oldest--she received some trust distributions and had less in the non-transferrable investment).
Our middle 2 kids also got federal refunds, but owed a nice chunk to state. We're telling them to adjust their withholding. In both cases, they earned a lot more this year over last year, and didn't see this coming. The youngest is getting $2 back. I hope he spends it wisely!