Occurs randomly. I believe it is only when there is a desync. It has happened to me twice on different versions and in different buildings.
[.50] The first time I died for an unknown reason when on the second story of a building. There were no "axe hitting zombie noises" (which occur when gunshots are far away) nor were there gunshots. My friend was in the same building, but could not find the body. I ran back to the spot and could not even find the ragdoll. I researched online and found out that other people had died without noise on the second floor as well, but did not understand the cause.
[.51] The second time I first noticed my screen flash black and white (like when receiving damage) then I noticed my shoes where ruined and my pants where damaged. I repaired my pants with a sewing kit, but around 5 seconds after that it happened again and this time my legs broke as well. That is when I logged out of the server since I heard no gunshots, remembering what had happened before. I rejoined a different server and was on the first floor (was still on second when I logged) with my legs still broken and my pants ruined. I was directly below where I was when I first received damage.
This second experience gave me enough information about the bug and I decided it atleast needed to be brought to the dev's attention.