Why was it important to change "FALLTHROUGH" to "fall through"?

[Bruce Korb via Gcc]

I don't write a lot of code anymore, but this sure seems like a
gratuitous irritation to me. I've been using

    // FALLTHRU and
    // FALLTHROUGH

for *DECADES*, so it's pretty incomprehensible why the compiler should
have to invalidate my code because it thinks a different coding
comment is better.