I would check over your return and see if you can see anything wrong especially in the payments section. Make sure you agree with all the amounts listed. Like is the difference the same or close to the refund you got last year? Maybe you put down that you applied last year's refund to this year? Or you put down you made a payment with an extension or paid an estimated payment that you didn't make?
If the IRS adjusted your refund they will be mailing you a letter explaining why the changed it.