What's a Snail Mail again? I think this is the first time I am hearing such term.

It is a mail delivered to people by traditional postal delivery service, so it literally means a mail.
I understood the scamming technique part. Also if my memory serves me right, I remember seeing a similar post of such scam a couple of months ago. But this snail mail name, how did it came to be...!
Yes, it was very common last year, Ledger users were the targets.
And how are they specifically targeting Trezor and Ledger users? Had there been any data leaks?
For the Ledger users, people that bought the hardware wallet had their home addresses or address they filled while buying the hardware wallet leaked to bad people. If the one for Trezor is true, it is very possible it is through leaked customers information also.