it's actually the version of the Magic-In-Motion marquee software the buses use. All those things like the GPS, the music and spiels on the buses, that's all part of MiM. Each version is named after a character with the first being MICKEY MOUSE and the second, I believe, being DONALD DUCK. Other versions have included REMY, SNOW WHITE, ARIEL, ARISTOCATS and more.
STITCH is the newest version which added features like the deadhead signs (DHPC for example) and the ability to announce special celebrations as guests board the bus.
since it's the version identifier, bus drivers really can't control which name goes up there.
I wrote a big article on it, I'll PM you the link.