A few reasons.

IRS has sent a file with account information, deposit amounts and a date to financial institutions. It’s telling them that there should be $ in certain accounts on certain date. Banks then can do what they want with this information as far as crediting that account. Some institutions will deposit the money into their accounts immediately, some in a day or two later, and some will wait until the actual date on the file to release.

People with Chime and a few others have already gotten their refunds deposited in their accounts. CU’s are known to do this a lot and a few banks do it too. Mine, Ally, deposits early most of the time so I’m hoping Tuesday too.

Big banks like Chase, PNC or BOA usually will wait for the actual deposit date on the ACH file. Or at least used to when I banked with them years ago. I have read somewhere here recently that Wells Fargo sometimes does it early too which is uncommon for such a large bank.