1) The PCs are Rangers, out exploring.
2) The PCs are Barbarians; they live here.
3) The PCs are Druids, wandering through nature.
4) The PCs are herbalists, out gathering rare plants.
5) The PCs are Wizards or Sorcerers, experimenting with a new, low-level teleportation spell, which dropped them off, here...
6) The PC was in a dungeon, last survivor of their old party, opened a scroll, read it, and... here they are!
7) Captured by (local bad-thangs), they managed to escape.
8) PCs encounter bad-thangs, find the new PCs tied or chained up, with their gear in the "treasure pile".
9) The PCs are repentant Clerics and/or Paladins, just finishing shriving their souls for some past misdeeds, and are now head back towards civilization, after a year of silence in the wilderness.
10) Escaped kidnap victims.
11) Beaten and robbed by bandits, brigands, orcs, blackguards, etc., and left for dead.
12) Lost and wandering... Took the wrong turn at Poughkipsie.
13) Adventurer! Out questing for fame, glory, and loot (in alphabetical order, not neccessarily order of importance).
14) Died; woke up here!
15) Amnesiac. Been wandering for a week, doesn't remember anything before that.
16) Attacked the evil Wicked Witch of the South, there was a bright flash, and... here we are!
17) Found this bag in a dungeon, See? Opened this bag, and pulled out a bean... Reached in, again, and these tentacles grabbed me, pulled me in, see? So I fought my way clear, and... here I was! Never seen anything like this! Why are all the leaves green?
18) Sailor on the Seas of Fate; shipwrecked on this strange world.
19) Was cursed to wander the Earth as a Shambling Mound for... who knows how long? Just came to herself!
20) Shapechanged into a cat, forgot she could change back. Chased mice for a while, then hit that patch of catnip... Don't remember much, for a while, after that!