i suppose it's superstition, but this is apparently quite a recent phenomenon, and pretty much only in america as far as i can tell. i've only ever seen people online say this.

in a way it's similar to some jewish people not pronouncing the word of G-d. (although i hear this is to avoid taking the Lord's name in vain - you can't take the name in vain if you never ever ever say or spell the name).

although the grand rising example is based on "morning" sounding like "mourning"... which i don't know, i'm not american or a new ager, but it sounds pretty silly to me.