Social Security can be up to 85% taxable depending on your filing status and other income. This means if you had social security benefits of $30,000 and you are married filing separately $25,500 would be included in your taxable income. If you are not having taxes withheld from that, then you would owe taxes at the end of the year as social security does not automatically withhold taxes based on your payment amount like an employer does.
If your situation will similar next year, then you can either make estimated payments or you can request from Social Security that they withhold taxes from your benefits.
Probably because some of the Social Security benefits became taxable, therefore increasing your taxable income.
Up to 85% of Social Security Retirement/Disability/Survivors benefits becomes taxable when all your other income plus 1/2 your social security reaches:
Look at your Form 1040 Line 6b for the taxable amount of your SS -
You can view your Form 1040 plus Schedules 1, 2 and 3 at any time using the online editions. Click on Tax Tools on the left side of the online program screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.