Gravity, I expect. Look at a picture of a crossbow. The crossbow string can be pulled back along the tiller to a locking mechanism which will hold the string in the fully drawn position. A bolt or quarrel is then placed atop the tiller and nocked onto the string.

As long as the prod (the bow part of the crossbow) is held horizontally, gravity keeps the bolt in place. Rotate the crossbow more than 45 degrees or so through the vertical, and the bolt falls off of the tiller.

If one stows one's crossbow to switch to an axe, spear, sword, or mace, is it not quite likely that it will be rotated more that 45 degrees through the vertical and the bolt will fall off the tiller?