Quests can be triggered by any number of characteristics - your
character's age, level, class, abilities - if you can think of it,
it's probably been a part of a quest somewhere. My personal favorite
are quests that are triggered by completing other quests, keeping the
story going.

I'm sure there are a good number of quests out there that have only
been seen by a select few, if any. It pays to be observant, it pays
to wander around. And sometimes it matters when you wander
around...