My husband is a 71 year old retired Airforce + disabled veteran.
He is the breadwinner. His income = our income.

Up until last year we got a tax refund every year.
Last year's refund was only $250.
This year it looks like we would owe $3,750.

Our income has not really changed except for small government issued increases.

What has changed (in tax regulations) to make our taxes go up $4,000 ?????

In general, in common sense,If you finish your tax return and are confused as to why you need to send the IRS a check, there is only one possible explanation for this: you paid less tax during the year than you owed for your income level. The reasons for this situation, however, can vary or your state may fully taxes pensions if you have pension fund income Without seeing both returns for both years it's not possible to say with any certainty.

NOTE: If your only source of retirement income is Social Security, then most likely you will pay no taxes in retirement. If you have other sources of income, a portion of your Social Security income is likely to be taxed. The amount of tax is determined by a formula. The result of the formula is that you may have to include up to 85% of the Social Security benefits as taxable income on your tax return. The amount that is taxable (anywhere from zero to eighty-five percent) depends on how much other income you have in addition to Social Security. Retirees with almost no income other than Social Security will likely receive their benefits tax-free and pay no income taxes in retirement. Most withdrawals from retirement accounts are taxed in retirement. This means IRA withdrawals as well as withdrawals from 401(k) plans, 403(b) plans, 457 plans, etc., are reported on your tax return as taxable incomeMost people will pay some tax when they withdraw money from their IRA or other retirement plans.
The amount of tax you pay depends on the total amount of income and deductions you have and what tax bracket you are in for that year