Perhaps they adventure because the grass is greener on the other side of the fence? Or maybe they just have a beef with somebody? Perhaps they find themselves on the horns of a dilemma. Or their backstory could be that they are retired adventurers not quite ready to be put out to pasture. Or maybe they are epic adventurers, the creme de la creme.
I know some of those ideas are whey too cheesy, but I thought I'd milk this idea till the cows come home. So to those who claim my post is udderly inappropriate, I say.... Bull!