Why was my pension reported as a ROTH?

There are two possible reasons why a pension distribution would be classified as a Roth IRA distribution on a tax return in TurboTax.  The first would be the pension fund administrator used the wrong distribution code in Box 7 of the 1099-R.  The second is that the wrong code was entered in to the software.  You should check your prior year 1099-Rs to see if they were properly coded.  If not, you should contact the pension plan administrator.  If the wrong code was entered, you will need to file amended returns for those years.  Alternatively you can wait for the IRS to catch the error and contact you.

It's doubtful that in previous years your pension was reported as Roth, although it might have been treated as nontaxable.  You can cause an otherwise taxable pension to be treated as nontaxable by misusing the Simplified Method.  Since you say that the entire pension should be taxable, you should not be using the simplified method at all.  You should just tell TurboTax that either the amount shown in box 2a of the Form 1099-R is the taxable amount or, if box 2a is blank, that you made no after-tax contributions and the entire amount is taxable.