Some people just do it all the time. The easiest simple move in the game-- every character can do it at almost any time-- is to hit the space bar and jump. And some people do it constantly.
Of course it drives some others nuts. 9 times out of 10, there's probably something better you could be doing than jumping-- even in an instance, you could be crafting, or summoning, or drinking for mana, or organizing the groups, or getting food and water ready, or organizing your bags. Why are you jumping? Stop it, you're giving me a headache!
Then again, I jump all the time. Usually, it's actually to signal impatience-- if I'm ready for a pull and we're not pulling, I'll start with the jumping. And if I ever feel helpless-- I've been spell interrupted, or I'm silenced, or in PvP, where there's nothing for me to do while I'm sitting stealthed defending by a flag. Some times when I'm surrounded and getting murdered in PvP, I'll jump just for the heck of it, like a last laugh against death (sometimes I even do the /laugh emote at the Paladin that's tearing me apart). Or sometimes I'm raidleading and I want to get the raid's attention. There are lots of good reasons for jumping around.
But many times, there's not, and I do it anyway. Are you a jumper?