It's been a normal word in the south forever. Outside of the south it's been pretty normal among black people. Black vernacular English has gained more prominence over the past few decades I think.

because it's gender neutral. similar thing with folks(/folx, ugh) In some cases it's also used to be condescending/ironic, plus s just love appropriating turns of phrase from black people

You is both singular and plural, back in old english there used to be "thou" but they decided they didn't need it... now they decided they needed it again idk

y'all is a perfectly cromulent word

It's a legitimately useful linguistic feature for a language to have a plural form of "you" so I fully support y'all becoming mainstream