Cause they got your SSN from the dark web. Come on, man, think!

643-03-7881

That's somebody's SSN maybe (I just typed random numbers and added dashes). Knowing an SSN alone doesn't enable a crook to do much of anything. They need to tie it to other information which proton presumably has. Now if I told you that number belonged to John Doe of Walla Walla, WA with this yahoo email and that September birthday, now an identity can be stolen with ease.

These sources report to Proton any time they find leaked information or data stolen in a hack from a third-party online service that’s tied to a Proton Mail email address or a Proton Pass alias.

I’m inclined to think that they look for known data breaches using your email address or alias, and if found they will tell you what kind of data is associated with the breach; e.g., social security, physical address, etc. So they don’t need to actually know that information, because they’re not searching using that information.

They probably just search for the information they already have and if somewhere your Name + Address and a SSN shows up it's quite likely that this is your SSN

Yeah thats pretty weird, maybe this is related to identities in proton pass? But they are not released yet