I recently received form 3531 from the IRS needing signatures on copy of 1040 and copies of W2s for my 2019 return that was e-filed via Turbo Tax Deluxe. Why?

If you can’t get to talk to someone at the IRS (which is likely) send them a signed copy of your 1040 with copies of your W-2’d as requested. We can’t know why they were requested. 

weird. that form is for mailed-in returns that are missing the required W-2 attachments and signatures.  actual signatures and w-2's are not required on an e-filed return. however, do what is requested. maybe your employer never filed the w-2's with the SSA.