Why did my paycheck withholdings increase in 2020 if I'm retired and don't get a paycheck?

Your paycheck withholdings would not increase if you are retired and do not get a paycheck.

The amount withheld depends on:

• The amount of income earned and
  • Filing status: Either the single rate or the lower married rate.
  • Number of withholding allowances claimed: Each allowance claimed reduces the amount withheld.
  • Additional withholding: An employee can request an additional amount to be withheld from each paycheck

From Tax withholding: How to get it right

The withholdings shown comes from sources other than a W-2, such as a 1099-R or SSA-1099.  But be sure you are entering those in the proper place, not as a W-2.