I have this exact same problem. I received 3 years of OPM Disability Retirement back pay in 2020. This should be pretty common, as OPM takes a long time to go through the approval process. If I could allocate this income to the years in which it should have been paid to me, I probably wouldn't owe any extra taxes this year. But if I claim it all in 2020, it bumps me up like 3 tax brackets and says I owe a lot. This is effectively penalizing the disabled for having to wait 3 years to receive the money owed to them.
