It's the cool new filler name; the -ay syllable is popular and one syllable names are easy to pair. I've seen Ray / Rey / Rae, May / Mae, Rayne / Rain / Reign, Jae, Kay / Kae, Fay / Faye / Fae, Layne / Laine / Lane, Grey / Gray. I even posted a Day in my last set of BAs. Popular for both genders too.