This has happened to me twice, and the 2nd time confirmed my theory.

The first instance this happened I had just logged in and maybe 1 minute prior to my log in found myself with broken legs. I spawned East of Electro and was inside one of the hospitals (the one on the coast), I was on the 3rd floor when all of the sudden my character grunted and fell to the ground. Legs broken. At the time I thought an agro'd zombie on the 1st floor had damaged me, breaking my legs. So I discarded it and respawned.

Several game hours after the first instance on another server, I had just logged in. Not a minute goes by and the same things happens. This time I logged in near the well on the hill by Electro. I ran to the fire station and was on the 2nd floor when it happened again. This time there were no zombies around (got into the habit of killing them before I run up stairs), and only 3 other people in the server. No shots were fired, just random damage. Legs broke and I crawled back to where I logged at.

The 2 common factors here are my character being above ground level in a building, and the breaking of the legs being close to my time of log in. I think it may have something to do with a lag spike or something which my character falls through the floor and then back to the original location and that breaks my legs. But I am not sure. I didn't have desync issues and my ping is consistently 15-30 on the servers I was on.

I have over 100 hours logged and this hasn't happened to me before.