As I've levelled (mostly legitimately I might add!) I have upon occasion had either an excuse or just felt like to do the following.
1. Spot AFKer at waypoint (or elsewhere).
2. Drag aggro mobs past AFKer.
3. Allow aggro mobs to down the player, kill mobs and res player for bonus XP.
It's completely opportunist douche-baggery, some will have done it by mistake, some probably haven't even realised they've done it. I do it on purpose for extra XP if the opportunity arises, I'll admit it
You can get a fair chunk of XP for doing this, sometimes worth 7 or 8 mobs worth of killing. Hardly tons, but highly amusing.