MY WIFE AND I ARE OVER 70 AND SHE WENT BACK TO WORK. WHY IS MY PENSION TAXABLE INCOME NOW FOR THE FIRST TIME?

Technically, your pension is generally always potentially taxable.  The difference is likely that her additional income increased your income above the standard deduction and possibly made your Social Security taxable and overall put your pension into being taxable as well.  

If your only income was say $20,000 in pension income and $20,000 in Social Security prior to her going back to work, then your total income would have not been taxable at all. 

To determine how much of your social security is taxable, you look at your total combined income.  If it is less than $32,000 your Social Security is not taxable.  If it is between $32,000 and $44,000 then half of your social security is taxable.  If your combined income is over $44,000 then 85% of your social security is taxable. 

 When calculating your combined income, only half of your social security is considered.  So in the case above none of your social security would be taxable so your total taxable income would be $20,000 because none of your social security would count.

This is below the standard deduction for a married couple filing jointly with both people over 65 would be $30,700 (standard deduction for 2023.

Now if your wife earned $25,000 while working, your social security would now be taxable at 85%
$20,000 from your pension plus $25,000 from her job plus 1/2 of your social security ($10,000) for a total combined income of $55,000

So your taxable income would be $55,000 minus $32,300 (the standard deduction for 2024) or $22,700.  

So using the example I made up, in 2023 you would have no taxable income, but in 2024, you would have $22,700 which makes it seem like all of your income is now taxable.