My friends and I have only ever experienced this bug while double gunning (gun on back and gun in hands) whilst logging out/in. Potential save for this is as soon as you log in and notice your naked is to log out immediately. This hasn't actually worked for us but I've heard of it working for some people. I can't say weather or not double gunning is definitively the reason but I'd say it's likely the main cause of it. It's pretty game breaking and should be remedied ASAP.

I had this issue just now. I alt + tabbed out while joining a server (never a good idea) However I was unable to re-tab back into Dayz, it only showed as a background process in Task Manager and Steam showed it as still running.