I was in the middle of scouting and for a lark decided to try and use a rock for my poacher ability "poison arrow" or "exploding arrow" because it seems like it would be really amusing to blow someone up with a rock. I thought that was what I was doing until I looked a bit closer and noticed that I was still holding the rocks, and my supply of arrows was going down as I shot them. They still had the range of a rock throw so I didn't notice initially. To test this I used all of my arrows exploding a cultist and then, sure enough, once I ran out of arrows and had just the two rocks in hand I could no longer use my poacher abilities that needed arrows.
So by holding a rock and having arrows in your quiver slot, you fire the arrows using poacher abilities.
Quite an amusing bug to find, but I'm sure that's not how it's supposed to work.