This is a very difficult question to answer without viewing your tax return but the only reason that makes any sense is that the nature of your income has changed. Either you have self-employment income this year that you didn't have last year or last year some part of your income was made up of long term capital gains and this year it was not.
If neither of these situations apply then there is something more that is wrong and you need to get a second set of eyes on your return before you file it.
@tommylindsey-ver