Much higher tax owed this year.

I'm attempting to prepare my 96 yr. old mother's taxes, which I did for her last year after my father passed away in Nov. of 2022.  Last year, their combined AGI was just over $109,000, and she received a refund of about $4,400.  This year, with her income from Social Security, Investments, and income from my Veteran father (DFAS), the AGI was $75,854, and it shows her owing $3,585.  This seems like a big difference, and I wonder if I've done something wrong.  Can anyone give me advice, or if further information is needed, I can provide it.  Thanks for any advice!