I've yet to confirm but I've been told that "autoloading the bolt off a corpse or from your inventory shortly before or after valuting one of those cute small fences, or running off a small height and hitting the ground like the stairs on those stilted houses" is the cause.

I've also been told "It's been a thing since they were added if you double tap R you can un-rack the bolt and re rack it, that's always fixed it for me in the past"