So I was walking through the woods when a random NPC called Wood Elf just popped out, summoned a Flame Atronach, and started shooting arrows at me. Killed them and got a nice Glass Bow, but I'm curious if anything provoked the encounter.

Wrong place at wrong time maybe. The encounters in this game are nuts, everything you come across attacks you or towns even if they are supposed to be passive. Loading screens say giants and bears do not attack unless provoked but with me in many instances, i am ambushed. Giants go at me like crazy even if i have my weapon sheathed and come no where close. Maybe the wood elf was having a bad day or trying to get the attention of the dark brotherhood too. Many times in the game the answer will come later in a story like a woman whos son went crazy, stole the family heirloom glass bow and was on a murder spree, i dunno, just my opinion