Usually happens on knockdown while aiming. Isn't really a big deal, just somewhat annoying. Dropping ALWAYS helped for me. The biggest annoyance I have with Xbows is when you die, bolts sometimes land directly on top of the Xbow preventing you to pick up the weapons. You'd have to pick up another Xbow somewhere, go back, pick up your bolts, then pick up the Xbow. This sometimes happens when you're standing still and drop your weapon as well.
