If you're riding Traveller's Tundra Mammoth or Grand Expedition Yak and you get melee'd on the back by a mob and get dazed, there's a chance for one of your npc passengers to get dismounted instead of you.
I tested it on both mounts, and sometimes I'd just be dismounted, and other times my passengers got dismounted before me from two separate dazes allowing me to run through mobs where I would have gotten dismounted twice. It seemed to be an equal chance for either me to be dismounted or one of my npc's to be thrown of when I got dazed. I was still slowed by the daze even if I wasn't dismounted by it though.
Proved to be pretty useful when I was leveling my second character to 90 not being able to fly in Pandaria.
tl;dr - Traveller's Tundra Mammoth and Grand Expedition Yak npc passengers can be dismounted instead of you when you get dazed.