None of us can answer that, since we users have no access to what you did for those years.
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Possibilities:
1) You had a Federal refund and "thought" some of that was going to pay your state taxes due? NOOOO, you can't do that. It's not allowed. You have to pay your state taxes entirely separately from what's on your Federal tax return.
2) You filed MD, but thought the MD tax was going to be paid by D.Debit or CrdtCd....but wasn't. Yoru proof of payment would be your own monthly Bank or CrdCd statements....maybe you put in the wrong account number, or maybe MD Doesn't allow a D.Debit when you e-file, and you had to pay MD separately....but failed to do so.
3) You filed MD both years, and paid, but failed to report ALL your income....MD discovered this and changed your tax return later to bill you for what you missed filing.
4) You (or spouse) worked in a neighboring Reciprocal tax state (VA, WV, DC, PA) and somehow messed that up....like claiming a tax credit for taxes paid to one of those 4 states (yeah, DC isn't a state, but is treated as one for tax purposes).
Without more details about exactly what happened, it's tough to hazard an accurate reason. You need to check thru what you filed, and what MD claims "in detail" . You need to make sure your original 2019 MD tax return was actually "accepted" first. ( 2020 is probably OK, since they couldn't have grabbed some of your 2020 MD "refund" if it hadn't first been e-filed and accepted. )
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Reasons #1 $ #2 above happen quite a bit every year to a number of folks who are confused about how payments work for their state.
(and... terminology: they didn't take your return....they took your "refund". A tax "return" is all the paperwork/data you send to them, irrespective of whether it is supposed to be a refund, or more taxes owed.)
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
